Auto merge of zcash#3228 - str4d:3058-taddr-encoding-refactor, r=str4d

Refactor t-address encoding

Includes code cherry-picked from the following upstream PRs:

- bitcoin/bitcoin#11117
- bitcoin/bitcoin#11259
  - Only the second commit (first is for QT code)
- bitcoin/bitcoin#11167
  - Only the first commit (the rest are not part of the t-address
encoding refactor).

Part of zcash#3058. Precursor to zcash#3202.