Optimize _ pattern in syntax-case clauses

[Jarhmander]

Plain _ patterns in syntax-case occurs fairly frequently in code (more than 80 occurrences in Chez Scheme itself), and these patterns never fail. There are used like else clauses for syntax-case.

An easy optimization is to avoid calling $syntax-dispatch for such patterns and directly embed the () return value that would result for such a call. A similar optimization already existed for patterns that consists of a single identifier.

With that optimization, cp0 can easily optimize out the last call to syntax-error that is never reached.

Before:

> (expand/optimize '(syntax-case x () (_ 'ok)))
(let ([tmp x])
  (let ([tmp (#3%$syntax-dispatch tmp '_)])
    (if tmp 'ok (#3%syntax-error tmp))))

After:

(expand/optimize '(syntax-case x () (_ 'ok)))
(begin x 'ok)

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Note: there is something fishy with the GitHub workflows, and one random job would fail. It happened on the main in my fork, as well as with my branch.

[burgerrg]

Thank you for this useful optimization!