Consolidate dependencies into a single repo?

[dgulotta]

The introduction pod crate uses a bunch of other crates for rsa, ecsdsa, eddsa, etc. This leads to two issues:

1. Sometimes we are using duplicate crates that serve the same purpose, e.g. the rsa crate uses my fork of plonky2_u32 while ecdsa/eddsa use ax0's fork.
2. Recently we forked plonky2, and this means that all of the dependencies need to be switched to the fork at the same time, which is tricky when different dependencies have different maintainers.

So perhaps we should have a single repository for all of these crates?