Parse error: Unexpected token for assignment: . for math on Go field

[SolarLune]

Risor seems to have an issue performing either -= or += operations on numerical fields contained within Go structs, though it can assign them fine.

For example, this doesn't work:

package main

import (
	"context"

	"github.com/risor-io/risor"
)

type Object struct {
	A int
}

func main() {

	src := "Object.A -= 1"

	_, err := risor.Eval(context.Background(), src, risor.WithGlobal("Object", &Object{}))
	if err != nil {
		panic(err)
	}

}

But this does:

package main

import (
	"context"

	"github.com/risor-io/risor"
)

type Object struct {
	A int
}

func main() {

	src := "Object.A = Object.A - 1"

	_, err := risor.Eval(context.Background(), src, risor.WithGlobal("Object", &Object{}))
	if err != nil {
		panic(err)
	}

}

EDIT: This is probably tied to #307.