Builtin address attribute on contract types

We talked about that instead of obtaining the address via `address(token0)` we could also obtain it via `token.address`. I think that would be inline with `self.address`, too. `address` would be a reserved word in the context of a contract properties. Since the ABI only defines functions it would still be allowed for a contract to implement an `address()` function though. We should probably create an issue and put a comment in for that, too.

_Originally posted by @cburgdorf in https://github.com/ethereum/fe/pull/179#discussion_r586575573_

No file, test, or entry point is identified in the issue. First clarify how contract properties should expose address, how address becomes reserved, and how an address() ABI function remains valid. Done should include agreed behavior for token.address, self.address, and the ABI exception, with tests covering those cases.

Written by the indexing model from the issue text.