Allow BidirectionalCollection.contains(subsequence:)

Allow `contains(subsequence:)` on any bidirectional collection instead
of just arrays, allowing usage with slices and arrays.

This replaces the Integer-based indexing with Index-based indexing.

Author: etcwilde

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -8323,7 +8323,7 @@ func assertString(
                 """, file: file, line: line)
 }
 
-extension Array where Element: Equatable {
+extension BidirectionalCollection where Element: Equatable, Index: Strideable, Index.Stride: SignedInteger {
   /// Returns true if the receiver contains the given elements as a subsequence
   /// (i.e., all elements are present, contiguous, and in the same order).
   ///
@@ -8337,10 +8337,15 @@ extension Array where Element: Equatable {
   {
     precondition(!subsequence.isEmpty,  "Subsequence may not be empty")
 
-    let subsequenceCount = subsequence.count
-    for index in 0...(self.count - subsequence.count) {
-      let subsequenceEnd = index + subsequenceCount
-      if self[index..<subsequenceEnd].elementsEqual(subsequence) {
+    guard self.count >= subsequence.count else {
+      return false
+    }
+
+    for index in self.startIndex...self.index(self.endIndex,
+                                              offsetBy: -subsequence.count) {
+      if self[index..<self.index(index,
+                                 offsetBy: subsequence.count)]
+                          .elementsEqual(subsequence) {
         return true
       }
     }