Convert bitcoin-tx and utilmoneystr to use Amount struct.

Summary:
This diff patches bitcoin-tx and utilmoneystr to use the
Amount struct instead of the previous CAmount typedef.

Test Plan: make VERBOSE=1 check

Reviewers: freetrader, deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D590