Remove these extra formatting traits, as they are
inconsistent with how arrays and slices format,
and it can cause unnecessary code bloat in binaries.
We can revisit this if people ever agree on doing these
formatters for the other slice-y types.
Prefer to dispatch to the `impl `fmt::Debug for [T]`,
to reduce the chances of monomorphizing twice.
Inlining it seems like a good idea for similar reasons?
Aligns module with rust-lang/library/core, creating an... unusual
architecture that is easier to pull in as a module, as core itself can
have no dependencies (as we haven't built core yet).
