Added gcc option '-Wconversion'

No warnings for standard numerical types. Still pending alternative
numerical types.

lstring.h

@@ -61,7 +61,8 @@ LUAI_FUNC void luaS_resize (lua_State *L, int newsize);
 LUAI_FUNC void luaS_clearcache (global_State *g);
 LUAI_FUNC void luaS_init (lua_State *L);
 LUAI_FUNC void luaS_remove (lua_State *L, TString *ts);
-LUAI_FUNC Udata *luaS_newudata (lua_State *L, size_t s, int nuvalue);
+LUAI_FUNC Udata *luaS_newudata (lua_State *L, size_t s,
+                                              unsigned short nuvalue);
 LUAI_FUNC TString *luaS_newlstr (lua_State *L, const char *str, size_t l);
 LUAI_FUNC TString *luaS_new (lua_State *L, const char *str);
 LUAI_FUNC TString *luaS_createlngstrobj (lua_State *L, size_t l);