Make UnsafeSNat et al. into pattern synonyms

...so that they do not cause coerce to bypass the nominal
role on the corresponding singleton types when they are imported.
See Note [Preventing unsafe coercions for singleton types] and
the discussion at #23478.

This also introduces unsafeWithSNatCo (and analogues for Char
and Symbol) so that users can still access the dangerous coercions
that importing the real constructors would allow, but only in a
very localized way.