Prerequisite work for openssl/project#1654. Add `OSSL_FN_rshift()` and `OSSL_FN_rshift1()`, wrap `BN_rshift()` and `BN_rshift1()` around them, and add focused tests for fixed-size right-shift semantics.
Prerequisite work for #1654.
Add
OSSL_FN_rshift()andOSSL_FN_rshift1(), wrapBN_rshift()andBN_rshift1()around them, and add focused tests for fixed-size right-shift semantics.