Feature request : More lightuserdata types

[云风 Cloud Wu <cloudwu@...>]

Lightuserdata is a value and all the lightuserdata share the same one metatable.

I think more lightuserdata type with independent metatable would be
helpful. I suggest changing some C APIs like these :

// Push a C pointer with type, returns a unique typeid (Not zero)
associated with this type.
int lua_pushlightuserdata(lua_State *L, void *p, const char *typename);

// Returns typeid if the value at the given index is a light userdata,
and 0 otherwise.
int lua_islightuserdata(lua_State *L, int index);

If we call lua_setmetatable with a  lightuserdata, the metatable of
different types are independent.

The total numbers of  types don't need to much, 64K (even 256) would
be a reasonable number, and it can be easily encoded in TValue struct.

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