Add default construct, and several ops, to Amount class
Summary:
- Add default constructor necessary for use with STL containers.
- Add division and modulus operators which are used routinely.
Test Plan: make VERBOSE=1 check
Reviewers: deadalnix, freetrader, #bitcoin_abc
Reviewed By: deadalnix, #bitcoin_abc
Subscribers: zquestz
Differential Revision: https://reviews.bitcoinabc.org/D549