Implement and test a few more arithmetic operations (mul, sqr, div)

[levitte]

Expectations:

• OSSL_FN functions for more arithmetic operations: mul, sqr, div
• Modify the corresponding BN functions to wrap around those OSSL_FN functions
• Additional tests added in test/fn_test.c

This is meant as a proof of concept for functions using OSSL_FN_CTX

[levitte]

Note: what constitues "arithmetic operations" are those mentioned in BN_add(3ssl), and distinguishes this issue from #1650 is that those implemented now are those who do use a BN_CTX.