[Cashtab] [p1 rename smallestDenomination] Rename fromSmallestDenomination to fromSatoshisToXec

Summary:
T2605

part 1 of stacked diff to rename fromSmallestDenomination and toSmallestDenomination functions

Renames fromSmallestDenomination to fromSatoshisToXec

Test Plan:
npm test
grep -r 'fromSmallestDenomination' src/ and verify no output

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D11850