Pass parent to computed properties of struct

[Haaxor1689]

This PR allows you to read properties not only from current context but from the parent context when writing a computed property of a struct.

const Foo = new r.Struct({
  type: r.uint32,
  data: new r.Struct({
    buff: new r.Buffer(64),
    values(_, parent) {
      switch (parent.type) {
        // ...
      }
    }
  })
})