Unify definedness binders for function applications

[JuanCoRo]

As per @tothtamas28:
When there are two identical function applications in a rule, separate binders for their definedness are generated.
It's possible that these two instances of the same application are not in the original KORE in this form, but through an alias.
The code generator inlines these aliases, so that's where the duplication might come from.

We could unify these definedness binders for ease of proving.