Support Luau Buffer type

[mxpv]

Implement support of luau's Buffer type.

Relevant lua API:

LUA_API void* lua_tobuffer(lua_State* L, int idx, size_t* len);
LUA_API void* lua_newbuffer(lua_State* L, size_t sz);
LUALIB_API void* luaL_checkbuffer(lua_State* L, int narg, size_t* len);

[mxpv]

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