runtime: Don't value copy structs from pointers on property access

Adds a test with a 10mb struct passed by a pointer, it would be previously
copied on property accessed whereas now the accesses just go through the
pointer without a copy.

Previous:
Benchmark_largeStructAccess-4          200     7914350 ns/op

After:
Benchmark_largeStructAccess-4      2000000         912 ns/op

bench_test.go

@@ -97,3 +97,25 @@ func Benchmark_call(b *testing.B) {
 		b.Fatal(err)
 	}
 }
+
+func Benchmark_largeStructAccess(b *testing.B) {
+	type Env struct {
+		Data [1024*1024*10]byte
+		Field int
+	}
+
+	program, err := expr.Compile(`Field > 0 && Field > 1 && Field < 20`, expr.Env(Env{}))
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	env := Env{Field: 21}
+
+	for n := 0; n < b.N; n++ {
+		_, err = vm.Run(program, &env)
+	}
+
+	if err != nil {
+		b.Fatal(err)
+	}
+}

vm/runtime.go

@@ -17,7 +17,16 @@ type Scope map[string]interface{}
 
 func fetch(from interface{}, i interface{}) interface{} {
 	v := reflect.ValueOf(from)
-	switch v.Kind() {
+	kind := v.Kind()
+
+	// Structures can be access through a pointer or through a value, when they
+	// are accessed through a pointer we don't want to copy them to a value.
+	if kind == reflect.Ptr && reflect.Indirect(v).Kind() == reflect.Struct {
+		v = reflect.Indirect(v)
+		kind = v.Kind()
+	}
+
+	switch kind {
 
 	case reflect.Array, reflect.Slice, reflect.String:
 		index := toInt(i)
@@ -37,13 +46,6 @@ func fetch(from interface{}, i interface{}) interface{} {
 		if value.IsValid() && value.CanInterface() {
 			return value.Interface()
 		}
-
-	case reflect.Ptr:
-		value := v.Elem()
-		if value.IsValid() && value.CanInterface() {
-			return fetch(value.Interface(), i)
-		}
-
 	}
 	return nil
 }