Factor bits to target conversion

Summary:
This diff introduces a new BNitsToTarget facility which avoids code duplication and can be used for heartbeat.

There is no change in behavior.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16574