Split the decoding of cashaddr in a pure decode step + a Destination creation step

Summary: This allows for much more in depth testing, even for destination we don't have support for.

Test Plan: Added a bunch of unittests.

Reviewers: dagurval, schancel, freetrader, #bitcoin_abc

Reviewed By: dagurval, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D652