[C++] Improve Allocator handling (#4312)

* Improve Allocator handling

- Templatize Allocator on vector_downward, and make it own the allocator
instance so it can manage lifetimes.
- Templatize + rename FlatBufferBuilderT accordingly, and add a typedef
to FlatBufferBuilder so old code continues to work.
- Fix some issues with the release deleter
- More details in github issue #4311

* Fix constexpr for older MSVC

* Reimplement allocator improvements via inheritance

Instead of templates, use an abstract base class and some unique_ptrs to
implement the new and improved allocator.

* Fix misplaced newline

* Add missing override keyword

* Add macro for func delete to support older compilers

* Explicitly move BufferDeleter (maybe fixes VS 10?)

* Revert previous attempt at VS10 fix

* Try yet another workaround for MS 10

* Use FLATBUFFERS_NOEXCEPT macro

* Add FLATBUFFERS_OVERRIDE macro

* Fix some issues with MSVC 16 hack

* Remove dep on unique_ptr

* Update DEPRECATED macro with a message

* Sigh, no delegating ctors before c++11

* Fix simple_allocator stub

* Relax FlatBufferBuilder ctor

* Add vector_downward reset + assert in clear

* Rename DetachedBuffer to FlatBuffer + remove unnecessary deprecations

* Add flatbuffers::FlatBufferBuilder::Release()

* Address remaining allocator-improvements comments

tests/test.cpp

@@ -83,7 +83,7 @@ void lcg_reset() { lcg_seed = 48271; }
 std::string test_data_path = "tests/";
 
 // example of how to build up a serialized buffer algorithmically:
-flatbuffers::unique_ptr_t CreateFlatBufferTest(std::string &buffer) {
+flatbuffers::DetachedBuffer CreateFlatBufferTest(std::string &buffer) {
   flatbuffers::FlatBufferBuilder builder;
 
   auto vec = Vec3(1, 2, 3, 0, Color_Red, Test(10, 20));
@@ -1520,11 +1520,11 @@ int main(int /*argc*/, const char * /*argv*/[]) {
   auto flatbuf = CreateFlatBufferTest(rawbuf);
   AccessFlatBufferTest(reinterpret_cast<const uint8_t *>(rawbuf.c_str()),
                        rawbuf.length());
-  AccessFlatBufferTest(flatbuf.get(), rawbuf.length());
+  AccessFlatBufferTest(flatbuf.data(), flatbuf.size());
 
-  MutateFlatBuffersTest(flatbuf.get(), rawbuf.length());
+  MutateFlatBuffersTest(flatbuf.data(), flatbuf.size());
 
-  ObjectFlatBuffersTest(flatbuf.get());
+  ObjectFlatBuffersTest(flatbuf.data());
 
   SizePrefixedTest();
 
@@ -1534,7 +1534,7 @@ int main(int /*argc*/, const char * /*argv*/[]) {
                        test_data_path;
     #endif
   ParseAndGenerateTextTest();
-  ReflectionTest(flatbuf.get(), rawbuf.length());
+  ReflectionTest(flatbuf.data(), flatbuf.size());
   ParseProtoTest();
   UnionVectorTest();
   #endif