Support serialization as another type without casting

Summary:
Backport of PR12731 (818dc74)

This adds a READWRITEAS(type, obj) macro which serializes obj as if it
were casted to (const type&) when const, and to (type&) when non-const.

This makes it usable in serialization code that uses a single
implementation for both serialization and deserializing, which doesn't
know the constness of the object involved.

Depends on D2929 and D2931

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Maniphest Tasks: T633

Differential Revision: https://reviews.bitcoinabc.org/D2932