Immutables

* Allow `None` to be used as key in pure-Python implementation.

Bugfixes

* Various improvements w.r.t. type annotations & typing
support (by hukkinj1)

* Fix pure-Python implementation to accept keyword argument
"col" correctly (by hukkinj1)

Bug Fixes
=======

* Fix the mutation API to maintain elements count correctly (25, 24)

* Don't allow MapMutation.update() calls after the mutation is finished

* Simplify MapMutation.__exit__() implementation

* Add new `MapMutation.update()` method that behaves like
`MutableMapping.update()`

* Make it faster to create a `Map()` from another `Map()`—it's now an
O(1) operation.

* `update()` method had a bug that could cause the update `Map` object to
have a wrong number of elements.

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.