Neither of these should have been feature flags, as they gate breaking
changes to the Zcash consensus rules (and in some ways are incompatible
with each other), while feature flags should be additive.
This inverts the dependency relationship between `zcash_protocol` and
`zcash_address`, permitting the network constants (primarily the HRPs)
defined in `zcash_protocol` to be used directly in `zcash_address`
instead of being duplicated.
Kris Nuttycombe
Jack Grigg