Manage librustzcash dependency about masp

[XuyangSong]

If we only depend librustzcash as zcash_primitives in masp_primitives. And in masp and masp_proofs, we use masp_primitives::zcash_primitives::* instead of zcash_primitives::*.
Would it be better?

[joebebel]

I don't think this really helps much. The benefit of importing the external zcash_primitives crate is that it is immediately obvious which code has been modified, whereas if we copy it into our repository, it requires manual diff to check this fact.

Actually, if anything, I would like to reduce the number of lines of code in our repository as it makes auditing and maintenance easier.