This code was part of Dolphin's relicensing from v2 to v2+ a while back,
we just never updated these copyright headers. I double-checked that
segher gave us permission to relicense this code to v2+ on 2015-05-16.
The loops relied on unsigned integer overflow, which is not immediately
obvious. Replace them with less clever variants that are clearer.
Also implement bn_compare using std::memcmp.
Pierre Bourdon