Inverse `Even` modulus

[pinkforest]

RSA has Even inverse modulus because p-1 for Euler's totient

• https://github.com/RustCrypto/RSA/pull/394/files#r1552977891

But only Odd is provided via BoxedUint::inv_odd_mod - similar to BY-GCD

• 590

I see @fjarri asked similar stuff before in math stackexchange:

• https://math.stackexchange.com/questions/4631776/modular-inverse-with-composite-even-modulus/4633908#4633908
• https://math.stackexchange.com/questions/3434557/how-to-solve-a-linear-congruence-8n9-equiv-0-pmod-1163/3434593#3434593

[fjarri]

I needed it for the same thing (inversion modulo totient), and the answer to that M.SE question is implemented in https://docs.rs/crypto-bigint/latest/crypto_bigint/struct.Uint.html#method.inv_mod . Not sure why it wasn't extended to BoxedUint, but it's probably quite straightforward.

[tarcieri]

Indeed that should be easy to port over

[tarcieri]

Fixed in #594