These served no type safety purpose; they were just to make developing
the crate easier. However, while docs.rs correctly shows these in trait
APIs as their plain `[u8; N]` form, editor LSPs like `rust-analyzer`
assume the crate-private types are public and auto-create stubs that
reference them, which is not good UX for downstream developers.
This, along with the corresponding `TryFromRawAddress` trait, enables
converting `ZcashAddress` into a network-agnostic type.
Closeszcash/librustzcash#564.
This enables the user-defined conversions to be fallible, which they
will almost always want to be (as address data needs to be validated
before it can be used).