NFC *except* that I noticed a bug by inspection where we suppress
`@escaping` when print enum element types. Since this affects
recursive positions, we end up suppressing `@escaping` in places
we shouldn't. This is unlikely to affect much real code, but should
still obviously be fixed.
The new design is a little sketchy in that we're using `const` to
prevent direct use (and allow initialization of `const &` parameters)
but still relying on modification of the actual object. Essentially,
we are treating the `const`-ness of the reference as a promise to leave
the original value in the object after computation rather than a
guarantee of not modifying the object. This is okay --- a temporary
bound to a `const` reference is still a non-`const` object formally
and can be modified without invoking UB --- but makes me a little
uncomfortable.
