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