Add CodeWriter utility class.

Helps simplify code generation code.  Instead of this:
  code += "inline const " + cpp_qualified_name + " *Get";
  code += name;
  code += "(const void *buf) {\n  return flatbuffers::GetRoot<";
  code += cpp_qualified_name + ">(buf);\n}\n\n";

You do this:
  code.SetValue("NAME", struct_def.name);
  code.SetValue("CPP_NAME", cpp_qualified_name);
  code += "inline const {{CPP_NAME}} *Get{{NAME}}(const void *buf) {";
  code += "  return flatbuffers::GetRoot<{{CPP_NAME}}>(buf);";
  code += "}";
  code += "";

Updated the CPP code generator to use the CodeWriter class.  Most of the
changes in the generated code are white-space changes, esp. around new
lines (since the code generator class automatically appends new lines
when appending a string).  Actual code changes include:

* Renamed "rehasher" to "_rehasher" for consistency with other args in
  Pack function.

* Renamed "union_obj" to "obj: in UnPack function.

* Always do "(void)_o;" to prevent unused variable warning in Create
  function (instead of only doing it if there are no fields) in order
  to avoid two-passes.

* Renamed padding variables from __paddingX to paddingX__.
  "Each name that contains a double underscore (_ _) [...] is reserved
   to the implementation for any use."  C++ standards 17.4.3.1.2.

* Add braces around switch cases.

* Calculate index as a separate statement in EnumName function, eg.
    const size_t index = ...;
    return EnumNamesX()[index];
  vs.
    return EnumNamesX()[...];

* Stored end table offset in variable in Finish() functions, eg.
    const auto end = fbb_.EndTable(start_, ...);
    auto o = flatbuffers::Offset<T>(end);
  vs.
    auto o = flatbuffers::Offset<T>(fbb_.EndTable(start, ...));

* Separate reinterpret_cast calls from function calls in Union
  functions, eg.
    auto ptr = reinterpret_cast<const T *>(obj);
    return ptr->UnPack(resolver);
  vs.
    return reinterpret_cast<const T *>(obj)->UnPack(resolver);

* Removed unecessary (void)(padding__X) no-ops from constructors, eg.
    Test(int16_t a, int8_t b) : ... {
      (void)__padding0;  // <-- Removed this line.
    }

In the idl_gen_cpp.cpp file itself, I refactored some code generation into
new functions: GenParam, GenNativeTable, GenVerifyCall, GenBuilders,
GenUnpackFieldStatement, and GenCreateParam.

Change-Id: I727b1bd8719d05b7ce33cbce00eb58fda817b25d

tests/namespace_test/namespace_test1_generated.h

@@ -1,5 +1,6 @@
 // automatically generated by the FlatBuffers compiler, do not modify
 
+
 #ifndef FLATBUFFERS_GENERATED_NAMESPACETEST1_NAMESPACEA_NAMESPACEB_H_
 #define FLATBUFFERS_GENERATED_NAMESPACETEST1_NAMESPACEA_NAMESPACEB_H_
 
@@ -21,11 +22,19 @@ enum EnumInNestedNS {
 };
 
 inline const char **EnumNamesEnumInNestedNS() {
-  static const char *names[] = { "A", "B", "C", nullptr };
+  static const char *names[] = {
+    "A",
+    "B",
+    "C",
+    nullptr
+  };
   return names;
 }
 
-inline const char *EnumNameEnumInNestedNS(EnumInNestedNS e) { return EnumNamesEnumInNestedNS()[static_cast<int>(e)]; }
+inline const char *EnumNameEnumInNestedNS(EnumInNestedNS e) {
+  const size_t index = static_cast<int>(e);
+  return EnumNamesEnumInNestedNS()[index];
+}
 
 MANUALLY_ALIGNED_STRUCT(4) StructInNestedNS FLATBUFFERS_FINAL_CLASS {
  private:
@@ -33,15 +42,28 @@ MANUALLY_ALIGNED_STRUCT(4) StructInNestedNS FLATBUFFERS_FINAL_CLASS {
   int32_t b_;
 
  public:
-  StructInNestedNS() { memset(this, 0, sizeof(StructInNestedNS)); }
-  StructInNestedNS(const StructInNestedNS &_o) { memcpy(this, &_o, sizeof(StructInNestedNS)); }
+  StructInNestedNS() {
+    memset(this, 0, sizeof(StructInNestedNS));
+  }
+  StructInNestedNS(const StructInNestedNS &_o) {
+    memcpy(this, &_o, sizeof(StructInNestedNS));
+  }
   StructInNestedNS(int32_t _a, int32_t _b)
-    : a_(flatbuffers::EndianScalar(_a)), b_(flatbuffers::EndianScalar(_b)) { }
-
-  int32_t a() const { return flatbuffers::EndianScalar(a_); }
-  void mutate_a(int32_t _a) { flatbuffers::WriteScalar(&a_, _a); }
-  int32_t b() const { return flatbuffers::EndianScalar(b_); }
-  void mutate_b(int32_t _b) { flatbuffers::WriteScalar(&b_, _b); }
+      : a_(flatbuffers::EndianScalar(_a)),
+        b_(flatbuffers::EndianScalar(_b)) {
+  }
+  int32_t a() const {
+    return flatbuffers::EndianScalar(a_);
+  }
+  void mutate_a(int32_t _a) {
+    flatbuffers::WriteScalar(&a_, _a);
+  }
+  int32_t b() const {
+    return flatbuffers::EndianScalar(b_);
+  }
+  void mutate_b(int32_t _b) {
+    flatbuffers::WriteScalar(&b_, _b);
+  }
 };
 STRUCT_END(StructInNestedNS, 8);
 
@@ -49,8 +71,12 @@ struct TableInNestedNS FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   enum {
     VT_FOO = 4
   };
-  int32_t foo() const { return GetField<int32_t>(VT_FOO, 0); }
-  bool mutate_foo(int32_t _foo) { return SetField(VT_FOO, _foo); }
+  int32_t foo() const {
+    return GetField<int32_t>(VT_FOO, 0);
+  }
+  bool mutate_foo(int32_t _foo) {
+    return SetField(VT_FOO, _foo);
+  }
   bool Verify(flatbuffers::Verifier &verifier) const {
     return VerifyTableStart(verifier) &&
            VerifyField<int32_t>(verifier, VT_FOO) &&
@@ -64,15 +90,20 @@ struct TableInNestedNSBuilder {
   void add_foo(int32_t foo) {
     fbb_.AddElement<int32_t>(TableInNestedNS::VT_FOO, foo, 0);
   }
-  TableInNestedNSBuilder(flatbuffers::FlatBufferBuilder &_fbb) : fbb_(_fbb) { start_ = fbb_.StartTable(); }
+  TableInNestedNSBuilder(flatbuffers::FlatBufferBuilder &_fbb)
+        : fbb_(_fbb) {
+    start_ = fbb_.StartTable();
+  }
   TableInNestedNSBuilder &operator=(const TableInNestedNSBuilder &);
   flatbuffers::Offset<TableInNestedNS> Finish() {
-    auto o = flatbuffers::Offset<TableInNestedNS>(fbb_.EndTable(start_, 1));
+    const auto end = fbb_.EndTable(start_, 1);
+    auto o = flatbuffers::Offset<TableInNestedNS>(end);
     return o;
   }
 };
 
-inline flatbuffers::Offset<TableInNestedNS> CreateTableInNestedNS(flatbuffers::FlatBufferBuilder &_fbb,
+inline flatbuffers::Offset<TableInNestedNS> CreateTableInNestedNS(
+    flatbuffers::FlatBufferBuilder &_fbb,
     int32_t foo = 0) {
   TableInNestedNSBuilder builder_(_fbb);
   builder_.add_foo(foo);