feat(library/scoped_ext): force user to end a scope with an identifier matching the one used in beginning of scope, closes #30

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): make sure all scopes are closed in the end of the module

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): make sure all scopes are closed in the end of the module