support some of GRIPS function notation

[mndrix]

GRIPS is another functional sugar for Prolog. Implement ideas from it, if they seem worthwhile:

• [ ] <- functional definitions that automatically insert cuts to make the predicate deterministic
• [ ] if guards for functional definitions

Perhaps these features should only be available by doing use_module(library(func), [grips/0]). Namely, explicitly enabling GRIPS support.