[bug#44518] gnu: Add matterhorn.

[elaexuotee]

Hey Guix,

This packages up a nice CLI client for Mattermost chat servers (a self-hosted,
libre Slack alternative).

Anyway, it's a Haskell package and ended up requiring quite a few
dependendencies along with a few version-updated alternatives to existing
packages.

The attached patch bundles all these up together, but what is the appropriate
convention in this case?

The bulk of the work is completely thanks to the hackage importer, but I did
spend quite a bit of time cleaning these up and sanity checking things.
Everything should pass the lint checks as well as indentation.

Also, for two packages, I had to disable tests. There are comments explaining
why in each case.

Let me know what you think!

0001-gnu-Add-matterhorn.patch
Description: Text Data