Don't unify "real" and dev dependencies

[tgeoghegan]

This commit moves the library and binaries targets to resolver = "2"
so that dev-dependencies don't get pulled in when building non-test
configurations of the prio library. Up until now, the crate releases
were enabling feature test-vector, unnecessarily including the
test_vector module and its dependencies. See 1 for details on
dependency resolution with the new resolver.

[tgeoghegan]

This one has an extra nuance that didn't turn up in divviup/janus#294: here, binaries/Cargo.toml turns on feature test-vector in its regular dependencies, not exclusively in its dev-dependencies. As a consequence, if we do cargo build --all, then prio gets built with test-vector because (AFAICT) features do get unified across the dependencies of all workspace members. However, I did verify with cargo build --verbose --lib and cargo publish --verbose that when building or publishing just the library crate, the test-vector feature does not get enabled.

[branlwyd]

I think this may be extraneous because it's implied by the workspace's resolver = "2". From https://doc.rust-lang.org/edition-guide/rust-2021/default-cargo-resolver.html:

The resolver is a global setting for a workspace, and the setting is ignored in dependencies. The setting is only honored for the top-level package of the workspace.