feat(shell): reactivate lean shell, only Lua frontend is working

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(util/script_state): replace splay_map with rb_map in the Lua environment

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(frontends/lua): reactivate some of the Lua extensions

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