Addresses are implemented in a non-obvious way

[Gusarich]

We should rework addresses, because currently the way they work is not intuitive and doesn't fit into how other types work.

Basically, addr_none should be a valid part of Address type and should be checked by devs against addressNone() (and not null). And Address? should mean Maybe Address, where Address also could be addr_none.

Original comment: #986 (comment)