Due to circular dependencies, it was necessary to temporarily split
`elliptic-curve` out of the workspace. It also can't currently resolve a
solution for `elliptic-curve`'s Cargo.lock due to `digest`. Excluding
`elliptic-curve`, all of the other crates work.

The main causes of this are crates like `sha2` and `sha3` which are
circular `dev-dependencies` (i.e. between traits and hashes).

This will temporarily break the `elliptic-curve` build until we can do
new releases of `sha2` and `sha3` as well as crates like `hmac` and
`hkdf`. Once these have been released, we can put everything back into a
single workspace.