Add : declarations for val definitions

[AlexKnauth]

Implements the declaration type annotations from #73.

(: f {Integer -> Integer})
(def f (λ [x] x))

(: fact {Integer -> Integer})
(defn fact 
  [[0] 1]
  [[n] {n * (fact {n - 1})}])

(: id (forall [a] {a -> a}))
(defn id
  [[x] (: x a)])

(: rmt (forall [a b] (Monoid b) => (Either a b)))
(def rmt (Right (: mempty b)))

There is one issue remaining, and that is the ambiguity between the expression version and the declaration version of : (including in the REPL and in Scribble examples). This could be solved by giving them different names.

[iitalics]

FWIW this patch seems to fix recursive definitions in the REPL.

Before:

> (defn f : {Integer -> Integer} [[x] (f x)])
; stdin:3:37: f: no inferred type
;   in: f
; [,bt for context]

After:

> (defn f : {Integer -> Integer} [[x] (f x)])
; (no error)

This also "fixes" recursive definitions in the documentation, but it breaks existing uses of : as expressions.

[lexi-lambda]

Thanks for the report; I’ve pushed a fix for the existing implementation that permits recursive definitions in the REPL.

[AlexKnauth]

Closing in favor of #86.