Slightly simplify some of the download implementation

See merge request !3

`cabal run tests` sometimes fails with an error about the build plan not
including the test suite :(  but we can make sure the plan includes the test
suite with this flag.

Basically, move more processing inside the `Either DownloadError` so that
context can be disregarded where it isn't relevant.

Also remove the list from `downloadShare` so that its caller can arrange for
any desired concurrency, retries, etc.

Incorporate tahoe-great-black-swamp to do share downloads using GBS

Closes #2

See merge request !2

StorageServer hides all of the networking details in the real implementation.
If tahoe-great-black-swamp works then the existing success case should
exercise all of the same logic in gbs-downloader whether we're actually using
it or not...

It might be nice to move all of the StorageServer implementations into
gbs-downloader and then test that they really do behave the same as each
other. Right now they're mostly in tahoe-chk though so that's annoying.

a bit, anyway

fixes for base64 issues

Try as hard as we can to make ``download`` return its failures in a Left
instead.

Also factor most of the tahoe-great-black-swamp parts out of `makeServer`in to
a separate helper function.  I thought this function might be where the
exception handling goes but it can't really yet because StorageServer requires
IO exceptions as part of its interface.  Should likely fix that sometime.

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