this makes `termination_by?` even slicker.
The heuristics is agressive in the non-mutual case (will omit `sizeOf`
if the argument is non-dependent and the `WellFoundedRelation` relation
is via `sizeOfWFRel`.
In the mutual case we'd also have to check the arguments, as they line
up in the termination argument, have the same types. I did not bother at
this point; in the mutual case we omit `sizeOf` only if the argument
type is `Nat`.
As a drive-by fix, `termination_by?` now also works on functions that
have only one plausible measure.
the user can now write `termination_by?` to see the termination argument
inferred by GuessLex, and turn it into `termination_by …` using the “Try
this” widget or a code action.
To be done later, maybe: Avoid writing `sizeOf` if it's not necessary.
