Get rid of exposed domain separation tags and use dedicated functions instead

[fjarri]

Fixes #28

As we have discussed, we need to keep the DSTs, but they're better off hidden in the respective functions.

What do you think, keep strings as tags, or use single bytes to make it a little more portable?

[cygnusv]

I think this PR will benefit from the discussion on #2 (comment)

[cygnusv]

what was I thinking 🙈

[fjarri]

I think this PR will benefit from the discussion on #2 (comment)

I think that discussion is kind of orthogonal to this PR, is it not? This is just code quality changes. One thing though that could be done is making the DST a parameter in the ScalarDigest constructor (leaving it to handle it however it sees fit, if we change the algorithm later). E.g.:

    ScalarDigest::new_with_dst(b"SHARED_SECRET")
        .chain_point(precursor)
        .chain_point(pubkey)
        .chain_point(dh_point)
        .finalize()