Add flake.nix support.

[RCoeurjoly]

When trying to build with flake, found two issues, which have been solved:

• Required tasty-hedgedog version not found
• showT not found in test/Parser.hs

Tested locally and it builds verismith without issues.

[ymherklotz]

Thanks, this looks good, and I've wanted to add support for flakes as well. Will try this out locally too and then merge it in the next few days

[ymherklotz]

Thanks again for adding the flake! Very useful. I've just added a devShell to the flake too so that you can use nix develop like nix-shell.

Sorry for the long wait.