Initial matcher/validator support

[timvermeulen]

Adds MatchingCollectionConsumer and MatchingCollectionSearcher as a matching/validating analogues of CollectionConsumer and CollectionSearcher:

public protocol MatchingCollectionConsumer: CollectionConsumer {
  associatedtype Value

  func matchingConsuming(
    _ consumed: Consumed,
    in range: Range<Consumed.Index>
  ) -> (Value, Consumed.Index)?
}

public protocol MatchingCollectionSearcher: CollectionSearcher {
  associatedtype Value

  func matchingSearch(
    _ searched: Searched,
    _ state: inout State
  ) -> (Value, Range<Searched.Index>)?
}

...as well as matching equivalents of the other consumer/searcher protocols.

Also adds a few algorithms that work with matches:

• firstMatch/lastMatch
• matches(of:)
• replacing(_:with:) using a closure:

let regex = Regex(OneOrMore(.digit).capture { Int($0)! })
let str = "foo 160 bar 99 baz"
print(str.replacing(regex, with: { match, _ in String(match.1, radix: 8) }))
// foo 240 bar 143 baz

[rxwei]

It would be great if we get additional examples of find-and-replace with a regex with multiple captures, both with the regex literal and with the result builder syntax. That will help inform our decisions around Swift tuples vs nominal tuples.

[timvermeulen]

@swift-ci Please test Linux

[milseman]

LGTM

[milseman]

This looks fine to me for now.

Outside scope of this PR, we might make RegexProtocol itself a consumer/searcher/matcher. But, I think it's best to establish parent-child relationships after we have the fundamental model in place. The important thing is getting this code and conformances in.

BTW, why does this need to be public?

[timvermeulen]

Some consumer/searcher algorithms return a type generic over the searcher, so the regex overloads return a type generic over RegexConsumer. So same story as PredicateConsumer at the moment.