std.crypto: Add public RSA module

[clickingbuttons]

RSA is still widely used for general cryptography. As such, it deserves a public module.

1. Move RSA functions from Certificate and tls.Client to new rsa module.
2. Add serialization
3. Fix side channel vulnerabilities
4. Add key generation

[voidastro4]

RSA-KEM[1] is useful to have, and due to arbitrary RSA key length it can serve as a hedge against PQC algos breaking by pushing quantum computer requirements higher.

[1] https://datatracker.ietf.org/doc/html/rfc5990

[jedisct1]

RSA-KEM[1] is useful to have, and due to arbitrary RSA key length it can serve as a hedge against PQC algos breaking by pushing quantum computer requirements higher.

Post-quantum safe RSA would require 1 terabyte keys. https://cr.yp.to/papers/pqrsa-20170419.pdf

Not exactly practical.

[voidastro4]

RSA-KEM[1] is useful to have, and due to arbitrary RSA key length it can serve as a hedge against PQC algos breaking by pushing quantum computer requirements higher.

Post-quantum safe RSA would require 1 terabyte keys. https://cr.yp.to/papers/pqrsa-20170419.pdf

Not exactly practical.

On the premise that no quantum computer could ever be constructed to crack them.

With MP-RSA and 1-10 MB keys you can buy decades at the very least.