Refactor IsCompressedOrUncompressedPubKey

Summary:
Using a switch is much more readable than a bunch of nested ifs.

Depends on D1568

Test Plan:

make check

Reviewers: #bitcoin_abc, kyuupichan

Reviewed By: #bitcoin_abc, kyuupichan

Subscribers: kyuupichan, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1569