Add ⋮ declarations for val definitions

[AlexKnauth]

This pull request adds the ⋮ form, which behaves like the : form in Typed Racket, or like the declaration variant of the :: form in Haskell. A Program like

(⋮ x τ)
(def x e)

is like this Haskell program,

x :: τ
x = e

which defines x with type τ to be equal to the value produced by e. It typechecks e with τ as the expected type just like if you wrote (def x : τ e).

This is similar to #74, but uses a different name so that : can be the expression variant even in "ambiguous" positions such as the REPL.

[AlexKnauth]

Closing in favor of #86.