Expose functionality of `lua_dump`

[alvinhochun]

From what I gathered, lua_dump allows for dumping the compiled bytecode of scripts which in theory can be loaded faster than the script in text form.

[jugglerchris]

This has been add as Function::dump() as part of #236. These can be loaded using the (unsafe) Chunk::into_function_allow_binary(). Another option is to use Lua::unsafe_new_with_flags() to create the Lua state, which lets you disable the wrappers around the Lua load family of functions which normally disable bytecode loading.