NOTE: this temporarily disables the `ecdh`, `hash2curve`, and `oprf`
features of `elliptic-curve` to make the release possible.

They can be re-enabled when the `hmac`, `hkdf`, `sha2`, and `sha3`
crates have been updated.

Going forward we can remove the `=` from the `digest` version
requirement now that we're done making major breaking changes, which
should allow for more flexible upgrades.