Prepare more files for explicit Amount constructor

Summary:
Continue T104 for:

• src/policy/fees.cpp
• src/primitives/transaction.cpp
• src/primitives/transaction.h
• src/script/sign.cpp

Test Plan: make check

Reviewers: deadalnix, freetrader, mbhinder, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D678