Slightly improve error message
Slightly improve error message

cross( (1,2,3), (4,5,6), :with(42) ) would die with the perplexing
error message: "Unexpected named argument 'with' passed".  This is
caused by the candidate that *does* accept :with has a Callable
constraint on it.  And since 42 is not a Callable, that candidate
is not selected, so the next candidate is, except for the fact that
that candidate does not accept a :with named parameter.

By removing the Callable constraint on the :with! candidate, the
error becomes: Type check failed in binding to parameter '&op';
expected Callable but got Int (42).  Still not pretty, but a lot
less perplexing.  Only other alternative would be to create an
additional candidate just to be able to create a more awesome
error message.  It's been done before, but this doesn't feel like
it is too much of a beginners trap.  Although a beginner found it:

  https://stackoverflow.com/questions/62202221/how-do-i-take-a-reference-to-new/62202837#62202837

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