Sapling address encodings
This PR enables Sapling keys and addresses to be passed in anywhere Sprout keys
and addresses are used. Doing so will cause crashes until those places are updated
with Sapling support.
Includes code cherry-picked from the following upstream PRs:
- bitcoin/bitcoin#11167
- Only the `ConvertBits()` function.
- bitcoin/bitcoin#11630
Closes#3058.