Moved swift4.0 support into single file

Sources/_SwiftSupport.swift

@@ -0,0 +1,42 @@
+import Foundation
+
+#if swift(>=4.1)
+#else
+  public extension Sequence {
+    func compactMap<ElementOfResult>(_ transform: (Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult] {
+      return try flatMap(transform)
+    }
+  }
+#endif
+
+#if swift(>=4.1)
+#else
+  public extension Collection {
+    func index(_ i: Self.Index, offsetBy n: Int) -> Self.Index {
+        let indexDistance = Self.IndexDistance(n)
+        return index(i, offsetBy: indexDistance)
+    }
+  }
+#endif
+
+#if swift(>=4.1)
+#else
+public extension TemplateSyntaxError {
+  public static func ==(lhs: TemplateSyntaxError, rhs: TemplateSyntaxError) -> Bool {
+    return lhs.reason == rhs.reason &&
+      lhs.description == rhs.description &&
+      lhs.token == rhs.token &&
+      lhs.stackTrace == rhs.stackTrace &&
+      lhs.templateName == rhs.templateName
+  }
+}
+#endif
+
+#if swift(>=4.1)
+#else
+public extension Variable {
+  public static func ==(lhs: Variable, rhs: Variable) -> Bool {
+    return lhs.variable == rhs.variable
+  }
+}
+#endif