Allow macros in classes

We should support macros defined in classes
```scala
class Num(x: Double) {
  inline def power(inline n: Long) = ~PowerMacro.powerCode(n, '(x))
}
object PowerMacro {
  def powerCode(n: Long, x: Expr[Double]): Expr[Double] = ...
}
```

The dificulty is that the contents of `PowerMacro.powerCode(n, '(x))` need to be placed in some static method.