Function clauses must match all possible inputs

[joews]

• [x] if expressions must cover all possible type values or have an "else" branch:

; passes type check
; should fail because arguments except 1 and 2 do not have a return value
(? 1 "one" 2 "two")

Fixed in #14

• [ ] function argument patterns must match all possible inputs

; passes type check
; should fail because not all inputs have a function body
(def f
  1 => "one"
  2 => "two")

; OK, because the name can be bound to any input value
(def f
  1 => "one"
  2 => "two"
  _ => "other")

This seems simple enough in this case, but I'm not sure how to do it for all lists:

; should fail
; but how do infinite types (e.g. Number) and finite types (Boolean) differ?
(def f
  (1|t) => "a"
  (h|(2|t)) => "b"
  (h|(i|'(1))) => "c")