Added functionality to assign field ids manually in a schema

New attribute:

-   `id: n` (on a table field): manually set the field identifier to `n`.
    If you use this attribute, you must use it on ALL fields of this table,
    and the numbers must be a contiguous range from 0 onwards.
    Additionally, since a union type effectively adds two fields, its
    id must be that of the second field (the first field is the type
    field and not explicitly declared in the schema).
    For example, if the last field before the union field had id 6,
    the union field should have id 8, and the unions type field will
    implicitly be 7.
    IDs allow the fields to be placed in any order in the schema.
    When a new field is added to the schema is must use the next available ID.

Change-Id: I8690f105f3a2d31fdcb75a4fab4130692b12c62f
Tested: on Windows

src/flatc.cpp

@@ -18,7 +18,8 @@
 #include "flatbuffers/idl.h"
 #include "flatbuffers/util.h"
 
-void Error(const char *err, const char *obj = nullptr, bool usage = false);
+static void Error(const char *err, const char *obj = nullptr,
+                  bool usage = false);
 
 namespace flatbuffers {
 
@@ -72,7 +73,7 @@ const Generator generators[] = {
 
 const char *program_name = NULL;
 
-void Error(const char *err, const char *obj, bool usage) {
+static void Error(const char *err, const char *obj, bool usage) {
   printf("%s: %s\n", program_name, err);
   if (obj) printf(": %s", obj);
   printf("\n");