Add random fuzzing of inputs

[gakonst]

ideally abstract over the backend so we can use both of the below:
https://github.com/AltSysrq/proptest
https://github.com/BurntSushi/quickcheck

instead using just the function selector as calldata, detect if the function has args and generate random inputs for N iterations
https://github.com/gakonst/dapptools-rs/blob/master/dapp/src/runner.rs#L104

[transmissions11]

should take some pointers from the echidna fuzzing engine which keeps a malicious input db and has what they call "constant mining" (recycles state vars and return data as input)

[transmissions11]

oh and shrinking would be rly helpful too

[gakonst]

I think PropTest already has shrinking https://github.com/AltSysrq/proptest/search?q=shrink, as implemented in #44

[transmissions11]

Can we keep open for adding advanced features like mutation or constant mining? Or should I crate a new issue

[gakonst]

Yeah let's get more specific issues open for that, along with labels for each.