Add _postCheck() function to OPCM

[maurelian]

          the current upgrade doesn't give us much that we can make assertions on, at least not without going into the nitty-gritty details (ie. checking all the getters, etc). 

We are not yet able to check init'd status on-chain.
We could check that semvers are set as expected?

At the least we should create an internal _postCheck() function in the OPCM which prompts developers to define whatever checks are appropriate while creating an upgrade.

Originally posted by @maurelian in ethereum-optimism/design-docs#173 (comment)