Note this requires test-based piecing together of the preamble string,
so it changed to being a std::string to make it easier to do.
This closes issue #254.
- Add new keyword int64_t/uint64_t/i64vec/u64vec.
- Support 64-bit integer literals (dec/hex/oct).
- Support built-in operators for 64-bit integer type.
- Add implicit and explicit type conversion for 64-bit integer type.
- Add new built-in functions defined in this extension.
Adds parseVersions.h as the base TParseVersions for versioning,
and splits the remainder between TParseContextBase (sharable across parsers)
and TParseContext (now the GLSL-specific part).
