Add type annotations

[J3RN]

Any expression should be allowed to have an associated type annotation. Haskell has a similar feature using ::, e.g.

(1 + 5) :: Int

Since functions are lambdas assigned to a name, we would also want this for lambdas (as Haskell allows):

(\x -> x + 1) :: Int -> Int

However I think making it postfix would have poor usability:

inc = (x) -> { x + 1 } :: (Int) -> Int

Ergo, I would suggest using a prefix notation:

inc = (Int) -> Int :: (x) -> { x + 1 }

I'm not super in love with this either. Haskell also allows a "declaration" above a definition, which might work better:

inc :: (Int) -> Int
inc = (x) -> { x + 1 }

I think this is rather good, but with the requirement that all expressions may be typed, we would need multiple syntaxes. Maybe the former can be stylized:

inc = (Int) -> Int
   :: (x) -> { x + 1 }

This could be acceptable.