Move sexpr_funcs to sexpr_fn. Using consistent file name conventions.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Use fullpath in #include directives.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Rename lean frontend files. The prefix lean_ is not necessary anymore.

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