Add an utility to convert string of bits from base 2^N to 2^M .

Summary:
This is extracted from D544 .

based on code from Pieter Wuille.

Test Plan: Added unit tests.

Reviewers: dagurval, #bitcoin_abc, tomtomtom7

Reviewed By: dagurval, #bitcoin_abc, tomtomtom7

Differential Revision: https://reviews.bitcoinabc.org/D572