[core] Implement Expression::serialize()

Issue #10714
- Each expression stores its operator as a string, and default serialization is [operator, serialize(child1), ...]
- Custom implementations of `serialize` for Expression types that don't follow the pattern
- expression::Value -> mbgl::Value converter
- node_expression bindings to expose `serialize`

platform/node/src/node_expression.hpp

@@ -32,6 +32,9 @@ private:
     static void GetType(const Nan::FunctionCallbackInfo<v8::Value>&);
     static void IsFeatureConstant(const Nan::FunctionCallbackInfo<v8::Value>&);
     static void IsZoomConstant(const Nan::FunctionCallbackInfo<v8::Value>&);
+
+    static void Serialize(const Nan::FunctionCallbackInfo<v8::Value>&);
+
     static Nan::Persistent<v8::Function> constructor;
 
     std::unique_ptr<Expression> expression;