This is pretty much a repackaging, because a lot has changed about
zcash in two major versions. Lots of libraries are no longer
necessary, and librustzcash is now part of the zcash package as
opposed to being a library supposed to be built seperately.
The whole thing is a buildRustPackage because that's easier than
trying to emulate buildRustPackage's dependency setup behaviour inside
a normal mkDerivation. So that the normal zcash build process is
followed, the actual configuring and building uses the normal stdenv
behaviour.
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