Add pretty-sha-path

pretty-sha-path provides minor-mode for prettifying Guix / Nix store paths by replacing SHA-sequences with ellipsis, i.e.:
```
/gnu/store/onecansee32lettersandnumbershere-foo-0.1  →  /gnu/store/…-foo-0.1
/nix/store/someother32lettersandnumbershere-bar-0.2  →  /nix/store/…-bar-0.2
```