Provide typed version fxpopcount functions

[cocodery]

This PR provides typed version fxpopcount functions, related to issue1319.

Before

Welcome to Racket v8.15-2024-12-25-34ef1af7ec [cs].
> (require racket/fixnum)
> (fxpopcount 42)
string:1:1: Type Checker: missing type for identifier;
 consider using `require/typed' to import it
  identifier: fxpopcount
  from module: racket/fixnum
  in: fxpopcount
 [,bt for context]

After

Welcome to Racket v8.15-2024-12-25-34ef1af7ec [cs].
> (require racket/fixnum)
> (fxpopcount 42)
- : Integer [more precisely: Nonnegative-Fixnum]
3

[github-actions]

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