refactor(bindings/lua/leanlua_state): cleanup

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(bindings/lua): move to frontends/lua

Lua API is an integral part of Lean. It does *not* have the same status
of external APIs (e.g., Python) we will add in the future.

We will reserve the directory bindings for external APIs for using Lean
as a library.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>