Rename library to vdaf

[branlwyd]

This repository is currently named libprio-rs, exporting a crate named prio.

This is a poor name: while libprio-rs does include a Prio3 implementation, it also contains trait definitions & functionality useful for generic VDAFs, as well as several other VDAF implementations.

For that reason, we should rename this library & crate to vdaf.

While working on this, we should consider how this change will affect existing library users. (We probably don't want to yank previous versions, but we should publish one final version of libprio-rs with a big deprecation warning pointing to the new vdaf library. Does Rust/Cargo provide any deprecation-warning functionality?)

[divergentdave]

The RustCrypto organization has handled name changes a few times, here's some artifacts from how they've done it.

https://docs.rs/crate/aes-ctr/0.99.99
https://sourcegraph.com/crates/aes-ctr@v0.99.99/-/blob/src/lib.rs
https://rustsec.org/advisories/RUSTSEC-2021-0061.html

[cjpatton]

Renaming is a good idea. Would it be desirable to have a more distinctive name? (Suppose someone else wanted to implement VDAFs in rust: It might not be "fair" for us to take the most generic name.)

[tgeoghegan]

It's too late for fair, I already squatted the name vdaf.

[divergentdave]

When we cut over, we should consider releasing the same code (except for Cargo.toml) twice, under the old and new names, to make things easy for cargo-vet auditors. If vdaf 0.1.0 is nearly identical to prio 0.LAST.0, then that makes it easy to justify adding an exemption for the 'new' crate.