disable gitlab caching for cabal files

See merge request !3

it's error prone, files go missing

parse the other SDMF directory capability types

See merge request !2

in particular, this helps ci

And switch to tahoe-capabilities' ConfidentialShowable for better dependency
directions / code sharing

Parse and serialize one known-correct example from Tahoe-LAFS

See merge request !1

At least somewhat.  We get all the fields out but we don't really interpret
most of them yet.