Update VM cheatcodes to be permanent

[anishnaik]

Closes #399

Before this PR, VM cheatcodes like warp, roll, fee, and coinbase only affected the VM state for the current transaction. Once the transaction ended, the effect of the cheatcode disappeared. This was a deviation from how foundry and echidna worked. Additionally, it prevented the use case of setting a "starting timestamp" in the constructor that can be used for the entire fuzzing campaign.

This PR allows for all these VM cheatcodes, except difficulty (see #547), to be "permanent". Meaning once the cheatcode is used, the actual block's value is changed which will affect all future blocks. If you change the timestamp, all future blocks will use that new timestamp as the starting point.

The primary implication of this is that the core invariants of the TestChain had to be changed. Block number and timestamp are no longer strictly increasing. Additionally, you can technically have two blocks with the same block number or timestamp. To handle these situations, we had to strip the TestChain for parts. This makes the whole implementation a lot simpler (and dumber). We no longer spoof blocks, assert that block number/timestamp is always increasing, and basically we no longer give a crap about trying to be a "real" blockchain.