Operating overloading does not work with functions defined using `expr.Function`

[purplefox]

If you attempt to overload an operator and the implementation of the function is defined using expr.Function as opposed to having the function in the env, then it fails to compile.

Please see #360 for more details

[antonmedv]

Here is the test that 🔴fails and needs to be fixed.

func TestOperator_Function(t *testing.T) {
	env := map[string]interface{}{
		"foo": Value{1},
		"bar": Value{2},
	}

	program, err := expr.Compile(
		`foo + bar`,
		expr.Env(env),
		expr.Operator("+", "Add"),
		expr.Function("Add", func(args ...interface{}) (interface{}, error) {
			return args[0].(Value).Int + args[1].(Value).Int, nil
		}),
	)
	require.NoError(t, err)

	output, err := expr.Run(program, env)
	require.NoError(t, err)
	require.Equal(t, 3, output)
}

type Value struct {
	Int int
}

Any volunteers? Tests should be added in https://github.com/antonmedv/expr/blob/master/test/operator/operator_test.go

[nikolaymatrosov]

I tried to implement functionality that passes the test in #408. It would be great if you can review the PR.

[Q16G]

Recently I was also troubled by this issue, I saw the issue, but I found that the new version does not seem to solve the problem