[==*] Extend ==* to represent ×.

[NoahStoryM]

Summary of Changes

See https://github.com/countvajhula/qi/issues/62 .

Since it is uncertain whether Qi should support arity modeling, this PR is just a draft.

Original:

$ racket profile/forms.rkt -f "relay*"
relay*: 126 ms

New version:

$ racket profile/forms.rkt -f "relay*"
relay*: 128 ms

[note] : this PR depends on racket/racket#4415 .

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