Wrap Lambda function in a struct to avoid crash in Mirror

Sources/HummingbirdMustache/String.swift

@@ -0,0 +1,23 @@
+
+extension String {
+    private static let htmlEscapedCharacters: [Character: String] = [
+        "<": "&lt;",
+        ">": "&gt;",
+        "&": "&amp;",
+    ]
+    /// HTML escape string. Replace '<', '>' and '&' with HTML escaped versions
+    func htmlEscape() -> String {
+        var newString = ""
+        newString.reserveCapacity(self.count)
+        // currently doing this by going through each character could speed
+        // this us by treating as an array of UInt8's
+        for c in self {
+            if let replacement = Self.htmlEscapedCharacters[c] {
+                newString += replacement
+            } else {
+                newString.append(c)
+            }
+        }
+        return newString
+    }
+}