Migrate PRF^Expand domain to a common Zcash primitives crate

[str4d]

/// The set of domains in which $PRF^\mathsf{expand}$ is defined.
/// This currently only includes domains used in Orchard.
/// TODO: refactor calls of $PRF^\mathsf{expand}$ for Sapling
/// to use this approach.

Originally posted by @daira in https://github.com/zcash/orchard/pull/93#discussion_r641922460

[str4d]

This would be perfect to go in a crate of Zcash primitives, which is what zcash_primitives was sort-of intended as. It has in practice grown significantly, so while I'd like that to be where it ends up, in practice it needs to go somewhere that can live underneath the orchard crate. We should either pick some underlying crate we can use in the meantime (maybe zcash_encoding), or implement this after refactoring zcash_primitives.