unifdef BN_RECURSION

unifdef BN_RECURSION

This removes a bunch of incomplete and scary code, which potentially leaks
secrets and is not constant time. A performance gain is achieved on arm64
for sizes that we care about, while a minimal decrease in performance is
noted for larger sizes on some other platforms.

While we will potentially reimplement Karatsuba (or Toom-Cook) at a later
date, it will be easier and safer to do it from a clean slate.

ok tb@