case-lambda allows multiple cases with the same arity

[I originally opened this as an issue for racket, but it more clearly violates the docs for typed/racket.]

Although for racket it doesn't seem explicitly forbidden, the typed/racket docs say:

Note that each formals must have a different arity.

Should this perhaps be an expansion-time error:

> (case-lambda [([z : Zero]) z]
               [([o : One]) o])
- : (-> Zero Zero)

This came up because a student encountering the case-> type constructor wondered whether case-lambda would provide overloading by type.