Add trait impls for core::cmp::Ordering

[tarcieri]

core::cmp::Ordering is #[repr(i8)], making it possible to impl the following traits for it by casting to i8 (and back, where appropriate):

• ConditionallySelectable
• ConstantTimeEq
• ConstantTimeGreater
• ConstantTimeLess

[tarcieri]

NOTE: I originally added this to upstream subtle in this PR: dalek-cryptography#102

I am curious about trying to modernize subtle to support things like const fn and asm! backends for emitting constant-time code, and thought perhaps subtle-ng might be an option for that which zkcrypto/ff and zkcrypto/group could potentially migrate to as work on upstream subtle seems stalled: zkcrypto/ff#148

So I thought I'd open this PR to test the waters.

[tarcieri]

It looks like the tests are currently failing due to an old MSRV:

https://github.com/tarcieri/subtle/actions/runs/21077404636/job/60622649261

  Downloaded ppv-lite86 v0.2.21
error: failed to parse manifest at `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/ppv-lite86-0.2.21/Cargo.toml`
Error: failed to parse manifest at `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/ppv-lite86-0.2.21/Cargo.toml`
Caused by:
  feature `edition2021` is required