Add .exists() and .iscontract() on to the address type

Not fully sure yet if `.exists()` can be implemented properly, but `.iscontract()` should be possible to be implemented via the `extcodesize` instruction.

With having `.iscontract()`, #4823 would not need to be implemented.

Begin with issue #4823 and the EVM extcodesize instruction cited in this issue. The payload names no files, tests, or entry points, so locate the address type implementation and its tests before assessing feasibility. Done means the semantics of .exists() and .iscontract() are decided and the supported behavior is implemented and tested.

Written by the indexing model from the issue text.