reduce closure size etc

Tell cabal to configure zlib a certain way when building it, making its build
errors go away without changing our own cabal package configuration.

`cabal list-bins ...` might output some garbage before outputting the path, so
this breaks :/  Also why do it the hard way when we can do it the easy way!

It fails because we didn't finish implementing it yet, and also because there
might be some more bugs.

Co-Authored-By:  <shae@private.storage>

I almost used it to narrow these two dependencies to library-only but it turns
out to not make much difference so it doesn't seem worth the extra complexity
now.

However, having access to the library might prove useful in the future.

Each generated with `cabal2nix https://.../` and then mangled by alejandro

somehow I forgot to write the test for this bit first
too exciting to actually be implementing a downloader I suppose

These give us the network protocol for downloading, the framing protocol for
decoding, and the cryptographic protocol for decrypting