rand_core v0.9 support

[tarcieri]

UPD: The migration is mostly complete. Remaining crates:

• kem: blocked on the hpke dev dependency
• elliptic-curve: blocked on
  • Bump to rust-random 0.9 zkcrypto/group#56
  • rand_core v0.9 support crypto-bigint#759

[tarcieri]

Edit: see original version of toplevel description for context

I think option 2 is probably the most straightforward. It's pretty much where we were before, and we have also added getrandom-based fallible APIs.

[newpavlov]

I think we can have two sets of methods: one set based on CryptoRng and another set of try_* methods based on TryCryptoRng. Note that fallible RNGs should implement the Try* traits, while potentially fallible ones can be converted to "infallible" using the UnwrapErr wrapper. Users then can decide what to use by themselves.

[tarcieri]

rand_core v0.9 is out

[baloo]

kem will need to be excluded from the workspace for a little while. It depends on x3dh-ke which depends on ecdsa. There is a bit of a release dance that will need to happen.

[tarcieri]

@rozbb perhaps we can get rid of the dev-dependencies of the kem crate now that its API has been massively simplified?