Immutables

New Features

* All new APIs are covered in the [README](https://github.com/MagicStack/immutables#api) file.

* Allow `Map` objects to be constructed from other mappings:
`Map(a=1)`, or `Map([('a', 1)])`, or `Map(dict(a=1))`.

* Implement `Map.update()` method.

* Implement `Map.mutate()` and `MapMutation` API.

* Make `Map` objects pickleable.

* Make `Map.keys()`, `Map.values()`, and `Map.items()` proper dict-view
like objects.

* Fixed a crash in Map constructor (C implementation)

* Reject keyword and positional arguments in __new__

* Breaking change: `Map.delete()` now raises a `KeyError` if a user deletes a missing key.

* Add a pure-Python implementation to support PyPy.

* Test coverage is now 100%.

* Implement ``Map.__repr__()`` and ``Map.__hash__()``