* First stab at implementing hashcode caching (#423)
Currently all existing tests pass but no cache_hash tests have yet
been added.
* Existing hash tests now pass on cache_hash classes
* Add towncrier change log
* Add documentation for cache_hash
* Fixes bug with check that init=True if cache_hash=True
* Fix long lines
* Fix documentation issues
* Add test for cache_hash requiring init
* Improve test coverage
* Remove now unnecessary 'pass'
* Add periods to the end of exception strings
* Add test docstrings for cache_hash tests
* Clarify documentation of cache_hash
* Recommend that hashable classes be frozen
* Fix test references for exception messages
3e0ecbd891