Support SRFI 273 return checks in SRFI 253

[aartaka]

Hello and thanks for integrating SRFI 253 into Gauche! I’ve made a follow-up SRFI to 253, SRFI 273. In it, one significant addition is procedure return type checks. In this PR, I’ve implemented it, cleaning up the code in the process.

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Another significant addition that’s likely to persist in SRFI 273 up to finalization is check-impl? auxiliary syntax. In short, it might allow providing implementation-specific types if standard predicates don’t fit the data. Something like

(check-arg (check-impl? <fixnum>) 3)
(define-checked (a [b (check-impl? <short>)] [c (check-impl? <short>)])
  (+ b c))
;; etc.

I’m not sure how to implement this one. It seems like check-arg is the most intuitive place to implement that, but I don’t want to pollute this generic macro with such specific details. What do you think?