fetch all the shares at the same time

Closes #11

See merge request !12

Try to ask for the right share number

Closes #10

See merge request !9

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