This is used when generating the Barrett reduction tables, and also when
applying the Barrett reduction at runtime, so we need it to be constexpr
friendly.
clmul() is a pure function and so should already be constexpr,
but it uses intrinsics that aren't defined as constexpr and
so the compiler can't really compute it at compile time.
Fix by defining a constexpr variant and dispatching based
on whether we're being constant-evaluated or not.
The implementation is simple, but in any case proof that it
is correct will be provided later on.
Instead of lengthy blurbs, switch to single-line, machine-readable
standardized (https://spdx.dev) license identifiers. The Linux kernel
switched long ago, so there is strong precedent.
Three cases are handled: AGPL-only, Apache-only, and dual licensed.
For the latter case, I chose (AGPL-3.0-or-later and Apache-2.0),
reasoning that our changes are extensive enough to apply our license.
The changes we applied mechanically with a script, except to
licenses/README.md.
Closes#9937
