[test_framework] Move hash160 to hash.py

Summary:
There's really no good reason to have it part of script.py. This is part of an effort to make script.py independent of the rest of the framework so we can use it in the plugin system.

In the future, we can move the other hash frunctions from messages.py to hash.py, too.

Test Plan: ninja check-functional

Reviewers: Fabien, #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16549