As a bonus, runGBS authenticates the server for us.

tahoe-chk in particular has a number of API incompatibilities, so also deal
with those.

And try more than one if necessary / possible

* make it single-param instead of multi-param
* get rid of its functional dependency

The cost is the use of another type family.  Seems like a win to me.

This makes share decoding open instead of closed!

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.

* check as many servers as necessary/possible for encoding parameters, instead
  of just the first
* add cap class instances for SDMF capability types
* teach the decoder about SDMF

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