SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.
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