Add BN#mod_sqrt

[btoews]

This PR adds support for BN_mod_sqrt as BN#mod_sqrt.

[rhenium]

BN_mod_sqrt() has a different function signature. https://github.com/ruby/openssl/actions/runs/3260413057/jobs/5353954966#step:6:88

../../../../ext/openssl/ossl_bn.c: In function ‘ossl_bn_mod_sqrt’:
../../../../ext/openssl/ossl_bn.c:537:47: warning: ordered comparison of pointer with integer zero [-Wextra]
  537 |  if (BN_##func(result, bn1, bn2, ossl_bn_ctx) <= 0) { \
      |                                               ^~
../../../../ext/openssl/ossl_bn.c:585:1: note: in expansion of macro ‘BIGNUM_2c’
  585 | BIGNUM_2c(mod_sqrt)
      | ^~~~~~~~~
../../../../ext/openssl/ossl_bn.c: At top level:
cc1: warning: unrecognized command line option ‘-Wno-self-assign’
cc1: warning: unrecognized command line option ‘-Wno-parentheses-equality’
cc1: warning: unrecognized command line option ‘-Wno-constant-logical-operand’

[btoews]

Whoops. Sorry about that. The signature matches BN_mod_inverse, so I defined a new macro for these two methods. I named the macro BIGNUM_2cr (2 BN args, context, returns result). Is that okay, or would you prefer something different?

[rhenium]

We probably need a Document-method: to tell rdoc about this method.

[btoews]

Added in 5befde7

[rhenium]

Yes, that makes sense!

[rhenium]

Merged, thanks!