OSSL_FN: stop wrapping BN functions around OSSL_FN - Update doc/designs/fixed-size-large-numbers.md

[levitte]

[levitte]

Update doc/designs/fixed-size-large-numbers.md to record the agreed change of direction. OSSL_FN is constant-time-always, BN keeps its own (fast)implementations, no function wrapping, convert at the top level.

See #2018 (parent issue) for related details