Refactor sapling_crypto::keys API to be more like orchard::keys

[str4d]

The keys module types are derived from some of the earliest sapling-crypto code. They have public internals, expose boundaries that don't make sense in end-developer applications, and are generally tricky to work with.

We took our learnings from Sapling, and crafted the orchard::keys APIs more consciously and carefully. We should refactor the sapling_crypto::keys API to match it (as much as is possible given the various differences).