Encode cashaddr size properly in version bit.

Summary: Encode the hash size in the version bit

Test Plan: make -j8 src/test/test_bitcoin && src/test/test_bitcoin --run_test='cashaddrenc_tests/*'

Reviewers: deadalnix, freetrader, dagurval, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D761