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New features:
* New verbosity level API for printing intermediate results and exceptions.
* New specifier for strings generated from a specified alphabet.
* Better error messages for tests that are failing because of a lack of enough
examples.
Bug fixes:
* Fix error where use of ForkingTestCase would sometimes result in too many
open files.
* Fix error where saving a failing example that used flatmap could error.
* Implement simplification for sampled_from, which apparently never supported
it previously. Oops.
General improvements:
* Better range of examples when using one_of or sampled_from.
* Fix some pathological performance issues when simplifying lists of complex
values.
* Fix some pathological performance issues when simplifying examples that
require unicode strings with high codepoints.
* Random will now simplify to more readable examples.