Prerequisite work for openssl/project#1654. Add `OSSL_FN_gcd()`, wrap `BN_gcd()` around it, and add focused tests for fixed-size greatest-common-divisor semantics.
Prerequisite work for #1654.
Add
OSSL_FN_gcd(), wrapBN_gcd()around it, and add focused tests for fixed-size greatest-common-divisor semantics.