add the/2 annotations from PlDoc comments

[mndrix]

PlDoc has a well-established format for adding type annotations. Convert these type annotations into the/2 declarations in each clause of the associated goal (when not in optimized mode). This will help during debugging and help verify the declared interface.

I prefer using PlDoc comments because:

• people already write them
• the format is tried and tested
• there's one comment per predicate, so type assertions aren't repeated in each clause

Unfortunately, PlDoc comments don't support type unions or type intersections. If a user wants that, he'll have to manually add the type assertions and describe the types in English in the docs.

This can be implemented by asserting user:term_expansion macros for each matching comment_hook. The macro just adds type assertions to the top of each clause body.

[mndrix]

This is finished. It needs some refinements but works pretty well.