include metadata in crash dumps.

Turns this repository into a cargo workspaces and adds a sub-crate with
the wonderful name 'simple-human-panic-test' that lives in the new
`tests/` directory. This should be of no consequence to the actual crate
itself or its deployment to crates.io, but allows us to easily test how
human-panic works when in an external crate. Especially, we can set up
the sub-crate `Cargo.toml` files in a way that we can easily test that
human-panic's output does not cause panics in humans.

For that, I've also added a simple integration test to the `simple` test
crate. It uses assert_cli to run the crate's main binary and assert its
output contains certain strings (as well as that it exits with a panic-
typical exit code). By the way: One benefit of using cargo workspaces is
that sub-crates share a lock file and a `target/` directory. So, we can
add lots of test crates without worrying of having to recompile
dependencies all the time.

You can easily run the tests across all sub-crates with
`cargo test --all`. In general, most cargo subcommands support `--all`,
and I've added it in a few places in the CI config.

for great ~~justice~~ formatting

…dding-dump-file

… into adding-dump-file