feat(frontends/lean): search for identifiers in the stack of namespaces; reject non-atomic names as local names

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): add '_root_' prefix for referencing names in the root namespace

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean): add '_root_' prefix for referencing names in the root namespace