Implement Recursive Transforms (#37)

* Implement Recursive Transforms

* Correct test names

* apply suggestions

* format

* add comments

* move the parse function

* refine `parseTransforms()` function

* refinements

* format

* Swift Format again

---------

Co-authored-by: Adam Fowler <adamfowler71@gmail.com>

Sources/Mustache/Transform.swift

@@ -48,7 +48,7 @@ public extension StringProtocol {
         case "uppercased":
             return uppercased()
         case "reversed":
-            return reversed()
+            return Substring(self.reversed())
         default:
             return nil
         }
@@ -66,7 +66,7 @@ private protocol ComparableSequence {
 extension Array: MustacheTransformable {
     /// Transform Array.
     ///
-    /// Transforms available are `first`, `last`, `reversed`, `count` and for arrays
+    /// Transforms available are `first`, `last`, `reversed`, `count`, `empty` and for arrays
     /// with comparable elements `sorted`.
     /// - Parameter name: transform name
     /// - Returns: Result
@@ -102,6 +102,78 @@ extension Array: ComparableSequence where Element: Comparable {
     }
 }
 
+extension Set: MustacheTransformable {
+    /// Transform Set.
+    ///
+    /// Transforms available are `count`, `empty` and for sets
+    /// with comparable elements `sorted`.
+    /// - Parameter name: transform name
+    /// - Returns: Result
+    public func transform(_ name: String) -> Any? {
+        switch name {
+        case "count":
+            return count
+        case "empty":
+            return isEmpty
+        default:
+            if let comparableSeq = self as? ComparableSequence {
+                return comparableSeq.comparableTransform(name)
+            }
+            return nil
+        }
+    }
+}
+
+extension Set: ComparableSequence where Element: Comparable {
+    func comparableTransform(_ name: String) -> Any? {
+        switch name {
+        case "sorted":
+            return sorted()
+        default:
+            return nil
+        }
+    }
+}
+
+extension ReversedCollection: MustacheTransformable {
+    /// Transform ReversedCollection.
+    ///
+    /// Transforms available are `first`, `last`, `reversed`, `count`, `empty` and for collections
+    /// with comparable elements `sorted`.
+    /// - Parameter name: transform name
+    /// - Returns: Result
+    public func transform(_ name: String) -> Any? {
+        switch name {
+        case "first":
+            return first
+        case "last":
+            return last
+        case "reversed":
+            return reversed()
+        case "count":
+            return count
+        case "empty":
+            return isEmpty
+        default:
+            if let comparableSeq = self as? ComparableSequence {
+                return comparableSeq.comparableTransform(name)
+            }
+            return nil
+        }
+    }
+}
+
+extension ReversedCollection: ComparableSequence where Element: Comparable {
+    func comparableTransform(_ name: String) -> Any? {
+        switch name {
+        case "sorted":
+            return sorted()
+        default:
+            return nil
+        }
+    }
+}
+
 extension Dictionary: MustacheTransformable {
     /// Transform Dictionary
     ///