Reduce buildBlock overloads with pairwise buildBlock(combining:into:).

This patch replaces `O(2^arity)` overloads of `buildBlock` with `O(arity^2)` overloads of `buildBlock(combining:into:)` with the language feature implemented in swiftlang/swift#40799. This improves library compilation time and fixes an issue where nested captures did not have a flattened type. This also allows us to switch back to native tuples for `Match`, which will be addressed in a follow-up patch.

Author: rxwei

Package.swift

@@ -42,14 +42,18 @@ let package = Package(
             name: "_StringProcessing",
             dependencies: ["_MatchingEngine"],
             swiftSettings: [
-                .unsafeFlags(["-enable-library-evolution"])
+                .unsafeFlags(["-enable-library-evolution"]),
+                .unsafeFlags(["-Xfrontend", "-enable-experimental-pairwise-build-block"])
             ]),
         .target(
             name: "_Unicode",
             dependencies: []),
         .testTarget(
             name: "RegexTests",
-            dependencies: ["_StringProcessing"]),
+            dependencies: ["_StringProcessing"],
+            swiftSettings: [
+                .unsafeFlags(["-Xfrontend", "-enable-experimental-pairwise-build-block"])
+            ]),
         .target(
             name: "Prototypes",
             dependencies: ["_MatchingEngine"]),
@@ -73,7 +77,10 @@ let package = Package(
         // MARK: Exercises
         .target(
           name: "Exercises",
-          dependencies: ["_MatchingEngine", "Prototypes", "_StringProcessing"]),
+          dependencies: ["_MatchingEngine", "Prototypes", "_StringProcessing"],
+          swiftSettings: [
+              .unsafeFlags(["-Xfrontend", "-enable-experimental-pairwise-build-block"])
+          ]),
         .testTarget(
           name: "ExercisesTests",
           dependencies: ["Exercises"]),