Function buf_init_size 

pub fn buf_init_size(
    state: &mut LuaState,
    buf: &mut LuaBuffer,
    sz: usize,
) -> Result<(), LuaError>

Initialize buf, reserve sz bytes, and return the writable region.

C: LUALIB_API char *luaL_buffinitsize(lua_State *L, luaL_Buffer *B, size_t sz)