Add covering function parameter patterns

[J3RN]

I think the only covering patterns are

• variable (e.g. a)
• record (e.g. {foo: foo})

The primitives (Int, Float, Char, and String) cannot have covering patterns; nor can ADTs with multiple constructors (including Lists). I could add pattern support for ADTs with only a single constructor, but this feels like it would be an incoherent addition; reconsider later.