Introduce BlockHash to represent a block hash

Summary:
So far we've been using raw uint256 for block hashes. Many things in bitcoin are uint256, such as transaction ids, merkle root, sighash, and so on.

Using a dedicated type ensures greater type safety and allows to make API more specific.

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4595