Support graybox fuzzing

[alastairreid]

Proptest has a great interface that provides a lot of control over the values that you fuzz with.
But (as far as I can tell), it is a blackbox fuzzer that cannot learn/use any knowledge of the code being fuzzed.
It would be good if a graybox fuzzing backend like AFL or libfuzzer could be used with the proptest API.

Arguably, this should be an enhancement request on the proptest project but I am putting it here because the whole idea of being able to easily switch between fuzzing and using formal verification is broken if, in practice, people are forced to use a different fuzzer because they need the added power of graybox fuzzing.