People frequently ask us how to pipe `spack find` output to other commands, and we tell
them to do things like this:
```console
$ spack find --format "/{hash}" | spack uninstall -ay
```
Sometimes users don't know about hash references and come up with potentially ambiguous
formulations like this:
```console
spack find --format {name}@{version}%{compiler} | spack uninstall -ay
```
Since this is a common enough thing to want to do, and to make it more obvious how, this
PR adds a `-H` / `--hashes` as a shortcut, so you can now just do:
```console
spack find -H | spack uninstall -ay
```
757f8ae59c