And try more than one if necessary / possible

new versions of both required for fixes involving incompatible API changes

This is clunky but it works ... for now.

Also add SDMF instances for them and tweak the tests to work with the new
tahoe-ssk encode interface.

a bit, anyway

base64 with ghc 8.6.5 on arm produces incorrect output

we need ghc 8.6.5 compatibility which means base 4.12

The requests time out for some reason but at least we're trying something.

Also sketch out a demo program

we need to use an _even older_ version of ghc than we're using now and
modern-uri doesn't seem particularly amenable to that.

network-uri is perfectly fine, anyway.

quite possibly both of these interfaces are wrong
may want to ponder them for a moment before proceeding

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

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>

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