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.

a bit, anyway

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

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

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