This PR closes #446 . It introduces the basic variant of startPrank and stopPrank cheat codes (msg.sender support only)

Expected behavior:

• startPrank does pranking only one call level deeper than it was initiated from. Thus, the scope startPrank is called in is not affected, only direct sub-calls.
• Multiple stopPrank can be called without a startPrank, with no issue/revert.
• Multiple startPrank can be called, overriding the previous startPrank (it does not act like a stack).
  • Foundry will error if a previous startPrank was not applied at least once. This doesn't seem important to us to enforce(?)
• A startPrank in a reverted scope should inherently have no effect (not be pranking anymore).

Outstanding TODOs:

• Update the cheat code wiki page.
• Open up issue to track adding support for tx.origin and delegateCall overloads in a later PR. The TX origin is exposed from evm.Origin, but we need an evm handle. And the delegateCall flag requires us to poke scopeContext.Contract.self, which is currently unexposed in medusa-geth. Relevant TODO comments are left as placeholders.