Make empty mapping a singleton

We might want to consider making the same optimization for empty mappings that CPython does for the builtin immutable types `tuple` and `frozenset`: make the empty mapping a singleton.

For reference, consider:
```python
tuple() is tuple()
# True

frozenset() is frozenset()
# True

import immutables
immutables.Map() is immutables.Map()
# False
```