Wrap Lambda function in a struct to avoid crash in Mirror

Sources/HummingbirdMustache/Lambda.swift

@@ -1,2 +1,36 @@
 
-public typealias HBMustacheLambda = (Any, HBMustacheTemplate) -> String
+/// Lambda function. Can add this to object being rendered to filter contents of objects.
+///
+/// See http://mustache.github.io/mustache.5.html for more details on
+/// mustache lambdas
+/// e.g
+/// ```
+/// struct Object {
+///     let name: String
+///     let wrapped: HBMustacheLambda
+/// }
+/// let willy = Object(name: "Willy", wrapped: .init({ object, template in
+///     return "<b>\(template.render(object))</b>"
+/// }))
+/// let mustache = "{{#wrapped}}{{name}} is awesome.{{/wrapped}}"
+/// let template = try HBMustacheTemplate(string: mustache)
+/// let output = template.render(willy)
+/// print(output) // <b>Willy is awesome</b>
+/// ```
+///
+public struct HBMustacheLambda {
+    /// lambda callback
+    public typealias Callback = (Any, HBMustacheTemplate) -> String
+
+    let callback: Callback
+
+    /// Initialize `HBMustacheLambda`
+    /// - Parameter cb: function to be called by lambda
+    public init(_ cb: @escaping Callback) {
+        self.callback = cb
+    }
+
+    internal func run(_ object: Any, _ template: HBMustacheTemplate) -> String {
+        return callback(object, template)
+    }
+}