refactor contract use of .env() considering `at()` api issues

[goastler]

at() rolls the contract storage back to block n, but does not do the same for calls to .env() in the contract. This means things like .env().block_number() will be the current block, not block n!

Consider this issue against the context of the function for each function in the contract. May need to make changes / alternate fns which accept the block number, etc, as a parameter instead