Allow cashaddr's prefix to be implicit

Summary: This makes for a much more pleasant UX.

Test Plan:
Added unittests.

In addition, try to send coin on testnet using both cashaddr with and without prefix and legacy addresses, with and without the -usecashaddr flag.

Reviewers: schancel, dagurval, #bitcoin_abc, Mengerian

Reviewed By: schancel, #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D839