The contents were always UTF-8 anyway, and &str has an equivalent representation
to &[u8], so this should not affect performance while removing unsafety at
edges.

It may be worth exploring a further adjustment that stores a single byte
(instead of 16) as the contents are always "", "-", or "+".

This prevents accidental dereferences and so forth of the Void type, as well as
cleaning up the error message to reference Opaque rather than the more
complicated PhantomData type.

Currently, function items are always tagged unnamed_addr, which means that
casting a function to a function pointer is not guaranteed to produce a
deterministic address. However, once a function pointer is created, we do expect
that to remain stable. So, this changes the show_usize function to a static
containing a function pointer and uses that for comparisons.

Notably, a *static* may have 'unstable' address, but the function pointer within
it must be constant.

Resolves issue 58320.