Rework bn_add()/bn_sub() to operate on word arrays.

Rework bn_add()/bn_sub() to operate on word arrays.

Rather than working on BIGNUMs, change bn_add()/bn_sub() to operate on word
arrays that potentially differ in length. This matches the behaviour of
s2n-bignum's bignum_add() and bignum_sub().

ok tb@