Don't panic when const-folding an unfoldable const expression

[LucaCappelletti94]

As with #1372 and #1373, this turns a fuzzer-found panic in the #[cube] macro into correct behavior, on the way to differential fuzzing once the front end is stable (see #1364).

The binary and unary const-folding paths in parse/expression.rs call as_const(context).unwrap() after checking is_const(). That assumes every const expression can be rendered back as a const token stream, but some cannot: a compiler intrinsic such as vectorization_of reports is_const() as true while as_const() returns None, so the macro aborted the user's build with an opaque panic. The fix falls back to a runtime expression when folding is not possible.

Reproducer: vectorization_of(x) + vectorization_of(x).

[nathanielsimard]

Just need to fix merge conflicts