Use Scala3's union type for stronger typing on |

[PapyChacal]

Hello, just a tiny signature-change that would improve our quality of life using fastparse in Scala3 🙂

Scala3 now has union types, which are perfect to express the type of the either-or parser combinator | but is unused in fastparse.

In our code, this leads to cases like:

class A <: Base
class B <: Base

def f(x: A | B): Something

def parseA: P[A]
def parseB: P[B]

def fancyParse: P[Something] = (parseA | parseB).map(
(x: Base) => f(x.asInstanceOf[A|B])
)

The asInstanceOf is required to bridge (parseA | parseB), currently typed to some common supertype, here P[Base], where it could now simply be typed P[A|B], which this PR offers 🙂 I just wanted to clarify the motivation, in case I missed an existing alternative or what not!

It sounds as non-intrusive as can be to me, not sure if I'm missing something though; similarly, I saw type tests specifically for implicit sequencers and everything, but none for the type of combined parsers. Tell me if you feel like this requires specific tests here!