Span the affected fields instead of reporting the field/variant name.

The Markdown engine used by the book can cope with a single leading space
on the list marker:

    Like this:

     * List item

    Rather than like this:

    * List item

… but it’s not the typical convention employed in the book, and moreover
the Markdown engine used for producing the error index *can’t* cope with
it (its behaviour looks like a bug, as it appears to lose one of the two
line breaks as well, but that’s immaterial here).

So, we shift to a single convention which doesn’t trigger bugs in the
Markdown renderer.

Renames TyTrait to TyDynamic.

The old parse code kept going even though it wasn't
supposed to, leading to an E0045 ("feature not allowed
on beta") printout

Diagnostics for struct path resolution errors in resolve and typeck are unified.
Self type is treated as a type alias in few places (not reachable yet).
Unsafe cell is seen in constants even through type aliases.
All checks for struct paths in typeck work on type level.

Fixes #29859

They are already gated with feature `fn_traits`