Adds `AstKind::debug_name()`, which returns the name of an AstKind
struct with a few minor details as applicable. I intentionally did not
make `AstKind` implement `Display`, as this method returns minimal
information. Additionally, this method only exists in debug builds. I've
hidden it behind a `#[cfg(debug_assertions)]`, so it won't affect
release builds.
This code was extracted from #642, where it as created while debugging
iteration over ancestor nodes. I figured it belonged in its own PR and
may be useful to other devs.