lua/uuid: rework luaT_{new,push}uuid() API

This change follows the previous commit regarding `luaT_newdecimal()`
and `luaT_pushdecimal()`, see explanation and details there.

Also changed the `luaL_` prefix to more appropriate `luaT_`. The `struct
tt_uuid` is our own type, the functions are specific to tarantool. So
`luaT_`.

Part of #7228

NO_DOC=refactoring, no user-visible changes
NO_TEST=refactoring, no behavior changes
NO_CHANGELOG=refactoring, no user-visible changes