Maybe allow nullary function syntax in definitions?

[stylewarning]

We allow (f) as a shorthand for (f Unit) but we don't support

(define (f)
  ...)

as a shorthand for

(define (f _)
  ...)

for Unit-taking functions.

The difficulty here is that it's not possible to determine at parse time that f takes Unit, unless it is declared as such.

Just posting this as an idea to see if others have thoughts.