expose DirectoryDownloadError for proper error handling in client applications

See merge request !10

And try more than one if necessary / possible

Add `downloadDirectory` function!

See merge request !8

I do not know why.

If you are not using nix then you'll have to figure it out yourself, sorrrrryy.

Turn WhichShare into an associated type family

See merge request !7

* 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!

Remove GitLab caching

See merge request !6

Support SDMF in the `download` API

Closes #8

See merge request !5

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.

* 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

Cleanup immutable factoring

See merge request !4