Deduplicate implementations of Hash160, and switch from deprecated crate ripemd160 to ripemd

[daira]

Hash160(x) is defined as RIPEMD-160(SHA-256(x)); we implement this both in zcash_primitives and zcash_client_backend. This might seem too trivial to deduplicate since it can be implemented in 3 lines, but that neglects that we want to minimize different versions of dependencies. See https://github.com/zcash/librustzcash/pull/472/files#r784831293 and https://github.com/zcash/librustzcash/pull/341/files#r784282022 .