Add global failure

[brockelmore]

Open to feedback on this, but it adds a global failure check if the cheatcode address has any code in it (dapptools and forge both do). This works by storeing a bool in slot 0 of the HEVM_ADDRESS & changing the failed to a private var with a custom view function that checks the slot. I was going to have it be a custom address, but forge uses a state changeset that only produces state changes if a contract has bytecode & without forge's etch cheatcode we can't do that here. I doubt anyone uses the HEVM address for storage like this anyway so I dont think it should be a problem. I could change the slot to bytes32("failed") if that would be better.

Some notes on implementation:

1. Have to hardcode HEVM_ADDRESS as an actual address in extcodesize call to satisfy 0.4.23 solidity semantics (constants not allowed in assembly blocks in that old version, but lifted in later versions)
2. I introduced a limited interface to the cheatcodes, primarily so users don't get confused between cheatcodes afforded by hevm and forge. I am willing to switch this to a low level call if the current implementation is annoying

Would allow for easy fix for dapphub/dapptools#768

[d-xo]

generally supportive, this seems like a nice fix to some legit issues.

I would slightly prefer the bytes32("failed") slot, and the low level call just to avoid cluttering up the namespace.

cc @MrChico and @rainbreak, any thoughts and opinions?

[gakonst]

ACK on the fix - indifferent on the low level calls / storage slot name. Whatever @d-xo @MrChico @rainbreak prefer

[d-xo]

got an out of band lgtm from @MrChico.

@brockelmore if you :

1. change the slot to bytes32("failed") (because we may want to store other stuff in HEVM_ADDRESS one day)
2. switch to a low level call

then I will merge 👍

[brockelmore]

Update for @MrChico:
Solidity 0.4.x doesn't support getting data from low-level calls. So we were left with either version bump or minimal interface. Speaking with @d-xo, it was decided a minimum version bump (to 0.5.0) was the better option (i agree fwiw). Should be good to go now imo. lmk if you guys want any styling changes

tested on all major versions (0.5.0, 0.6.0, 0.7.0, 0.8.0)

[d-xo]

Thanks!