Enable FFI cheatcode

[montyly]

By default HEVM creates a VM where vmoptAllowFFI is set to false:

https://github.com/dapphub/dapptools/blob/a2b96cc2dbc28508c9fe699d6438bf6eccafc2ad/src/hevm/src/EVM/Exec.hs#L21-L46

We can add a flag in Echidna to set this to true (or just always enable it to true if it does not impact performance).

[patrickd-]

Rather than impacting peformance, true-by-default may impact developer's local security, so a flag would probably be preferable

It is generally advised to use this cheat code as a last resort, and to not enable it by default, as anyone who can change the tests of a project will be able to execute arbitrary commands on devices that run the tests. (from foundry docs)

[gustavo-grieco]

I'm working on this in #750, it should work if you want to test it.

[gustavo-grieco]

Already in master.