Fix compile errors in Amount caused by ambiguous instantation

Summary: Amount(long long) was ambiguous causing compile errors. Instantiation is now generalized to all integers

Test Plan: None; minor refactoring only

Reviewers: schancel, O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: schancel, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D594