`define-typed-syntax` doesnt handle implicit `:` with multiple \Rightarrow's

[Stephen Chang]

Eg (in fomega.rkt), the inst rule below needs the explicit : after the ⇒.

(define-typed-syntax (inst e τ ...) ≫
  [⊢ e ≫ e- ⇒ : (~∀ (tv ...) τ_body) (⇒ (~∀★ k ...))]
  [⊢ τ ≫ τ- ⇐ k] ...
  #:with τ-inst (substs #'(τ- ...) #'(tv ...) #'τ_body)
  --------
  [⊢ e- ⇒ τ-inst])

Otherwise I get the exception:

; datum->syntax: contract violation
;   expected: (or/c syntax? #f)
;   given: ...

[Stephen Chang]

define-typed-syntax: properly handle implicit : with subsequent ⇒-props

• fixes #33

→ <<cset c9c0970307e7>>