* 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
* Add narrative chapter on initialization
* Be more explicit about defining validators both ways
* Stress that the name of default/validator != attr name
* Mention nested schemas
* Explain handling of private attributes
* Add another consequence of _ stripping
* Stress that nothing should overwrite attributes on class body
* Better wording
* typo
* Dedup examples
* Address review feedback
* Add newsfragments
Ryan Gabbard