Older versions of GCC (at least on ARM) complain about narrowing conversions and
overflows when setting the reference index (defined as an 11-bit signed integer
bitfield) to -1:
```
error: narrowing conversion of '-1' from 'int' to 'short unsigned int' inside { } [-Wnarrowing]
error: conversion from 'short unsigned int' to 'short unsigned int:11' changes value from '65535' to '2047'
```
This is due to the signedness of integer bitfields (not explicitly signed or
unsigned) being implementation-defined. This addresses this issue by explicitly
defining the signedness of all the bitfields in `flatbuffers::TypeCode` in C++.
* [Python] Codegen SizeOf classmethod for structs
This codegens a `SizeOf()` classmethod for all structs since we can't
determine the size of a FlatBuffer generated struct from Python otherwise.
* [JS/TS] Codegen sizeOf static method for structs
This codegens a `sizeOf()` static method for all structs since we can't
determine the size of a FlatBuffer generated struct from JavaScript or
TypeScript otherwise.
This optionally generates a static `getFullyQualifiedName()` function to get
the fully-qualified name of a type in JavaScript and TypeScript in a similar
fashion to the C++ codegen.
Anass Al