Wrap the BIGNUM type around the OSSL_FN type

[levitte]

Expectations:

• The BIGNUM type uses the OSSL_FN type instead of having dmax and the d array directly
• All BIGNUM expansion functions are adapted for these changes
• Optionally, top is removed from BIGNUM, ensuring that a large number will never shrink its memory usage

[levitte]

Experience shows that I need to maintain dmax, d, etc for now, to not drive myself crazy. I'm marking them for removal in comments, so when we're ready for it, they should disappear.