Upgrade at least to `rust-secp256k1` 0.21

[thomaseizinger]

The 0.21 release will include rust-bitcoin/rust-secp256k1#326 which is required for interoperability with the ThirtyTwoByteHash trait. In order to use rust-secp256k1-zkp in a project together with rust-secp256k1 0.21, we will need to bump the version we are depending on and release a new version as well.

[thomaseizinger]

Looks like we have a release: rust-bitcoin/rust-secp256k1#347

[apoelstra]

We need to do a point release (probably today) beacuse we added a new type but forgot to export it publicly.

[thomaseizinger]

Yeah I saw that. Shouldn't matter much for this repository though because we don't pin the version to the patch release so I am going to go ahead and prepare a patch to bump to 0.21.