Rename `alr list` to `alr crates` (#671)

* Rename `alr list` to `alr crates`

This way there is less confusion about the purposes of `alr crates` and `alr
search`; the former operates on crates and the latter on releases.

* Fix bash completion

* Document user-visible changes

* Remove `crates` command, move to `search --crates`

* Update bash completion for new situation

* Update user-facing document

* Updated testsuite