feat(frontends/lean): add coercion modifier

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): check whether namespace exists or not in the 'using' command, add to_valid_namespace_name helper function

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): check whether namespace exists or not in the 'using' command, add to_valid_namespace_name helper function