teach the/2 about intersection types

[mndrix]

Intersection types are just a conjunction of two other type constraints. We see their utility in the default types implemented in error:has_type/2. For example, positive_integer is really the intersection of positive and integer.

Support intersection types with a syntax similar to union types:

the((positive,integer), X)
the((nonempty,list), Xs)

It might be nice to add a little extra sugar so we avoid the extra parens, but I'm not we should at this point:

the(positive, integer, X)