Remove compilation bottleneck

Benchmarks on Xcode 12.4 showed compiling / type checking
StrideCollection.offsetBackward was the bottleneck for building
the package. Replacing the ternary operator with the longer
form, removes the bottleneck.

Author: iainsmith

Sources/Algorithms/Stride.swift

@@ -226,9 +226,14 @@ extension StrideCollection: Collection {
     offsetBy n: Int,
     limitedBy limit: Index
   ) -> Index? {
-    let distance = i == endIndex
-      ? -((base.count - 1) % stride + 1) + (n - 1) * -stride
-      : n * -stride
+    // We typically use the ternary operator but that stresses out the compiler.
+    // To avoid slow build times for consumers, we use the longhand below.
+    let distance: Int
+    if i == endIndex {
+      distance = -((base.count - 1) % stride + 1) + (n - 1) * -stride
+    } else {
+      distance = n * -stride
+    }
     return base.index(
         i.base,
         offsetBy: distance,