Switch to clap-generated completions

The current completions required a lot of domain-specific knowledge
about each individual shell and their completion functionality. Much of
which is sparsely documented.

While clap does not generate perfect completions, since parameters like
`-e` are missing completions, it does a reasonable job while requiring
no work on writing these completions.

Since access to `cli.rs` isn't possible from the `build.rs`, these
completions aren't always generated on build. Instead a test verifies
that there has been no changes to these completions and provides a
simple code sample for re-generating them. This should provide a simple
solution with minimal overhead.

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