proptest 0.6.0

Hypothesis-like property-based testing and shrinking.

# Acknowledgements

This crate wouldn't have come into existence had it not been for the [Rust port
of QuickCheck]( and the
[`regex_generate`]( crate which
gave wonderful examples of what is possible.

## Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall
be dual licensed as above, without any additional terms or conditions.