Allow to specify which user calls each method in a contract about echidna

[japesinator]

This is pretty easy to fix when API scripting, just write some MonadGen m => m Address that picks from a list of addresses you care about, then add state . caller .= a where a is the result of that generator to your Actions.

Totally open to adding it to the CLI, and it shouldn't be all that hard, but unsure what API to use. Thoughts? If someone submits an RFP or whatever, I'll try to get it done this weekend.

[ggrieco-tob]

So one possible approach to start is to use a value generator that produces the address to use as caller in the EVM transaction. For instance, the signature of execCall could be:

execCall :: MonadState VM m => (Address, SolCall) -> m VMResult

What do you think?

[ggrieco-tob]

This is already implemented in echidna and perprop.