+ a bunch of tests for vm.addr()
+ move the known_keys and known_sigs to the Exec object, making it part of cheatcodes meant that tests could interfere with each other

Still a WIP, the output of vm.sign() is not connected to ecrecover() yet

avoids relying on counters for name collisions and repeated vm.addr(x) applications are implicitly equal, no need to add explicit constraints

(under the right circumstances)

calling vm.sign(key, digest) will now:
- generate (v, r, s)
- generate a constraint that ecrecover(digest, v, r, s) == vm.addr(key)
- generate the same constraint for the equivalent signature

this is simpler and more efficient than the previous way to do it entirely as constraints between ecrecover calls