Added startPrank() and stopPrank() cheatcodes to hevm.

[alecmaly]

Description

Added startPrank() and stopPrank() cheatcodes to hevm.

This should assist in testing smart contracts when using tools that rely on hevm such as Echidna or Medusa.

Auditors are creating complex proxy patterns to do external testing of contracts using tools such as Echidna / Medusa: How to set up Multi Actor Invariant Testing

Note Alex The Entrepenerd's response in Discord stating startPrank() would eliminate the need for this testing pattern: link to discord conversation

Testing

Tested locally with

cabal test --test-options='-p /Cheat-Codes-Pass/'

Look for tests prove_startPrank(address) and prove_startPrank_val()

Checklist

• tested locally
• added automated tests
• updated the docs
• updated the changelog

Additional Notes

Edge cases such as stacks of startPrank() were not tested/considered.
For example, this code will not work - although I am unsure how this operates in Foundry anyway.

.. code 1 ..
hevm.startPrank(address1);
 .. code 2 ..
hevm.startPrank(address2);
 .. code 3 .. 
hevm.stopPrank();
.. code 4 ..
hevm.stopPrank();
.. code 5 ..

While untested, I would expect:
code 1 to have msg.sender = default
code 2 to have msg.sender = address1
code 3 to have msg.sender = address2
code 4 to have msg.sender = default
code 5 to have msg.sender = default

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I have never written code in Haskell before nor do I have experience with this codebase, please let me know if there are edge cases to consider or any other styling I should do to conform to this repo's standards.

[arcz]

LGTM!