Add SipHashUint256Extra as a specialization of siphash for a 256bits int and a 32 bit one.

Summary:
This is going to be used to hash outpoints. This is part of the work started with D342 .

Depends on 490

Test Plan:

make check

Reviewers: freetrader, CCulianu, #bitcoin_abc

Reviewed By: CCulianu, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D491