This repr-hint makes a struct/enum hide any niche within from its
surrounding type-construction context.
It is meant (at least initially) as an implementation detail for
resolving issue 68303. We will not stabilize the repr-hint unless
someone finds motivation for doing so.
(So, declaration of `no_niche` feature lives in section of file
where other internal implementation details are grouped, and
deliberately leaves out the tracking issue number.)
incorporated review feedback, and fixed post-rebase.
88747ffe63