Pass scalar by reference in secp256k1_wnaf_const()

Summary:
After this change, no struct or union is passed by value in the
entire codebase. This makes it easier to compile the library with
CompCert.

This is a backport of secp256k1 PR617

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4994