priv impls do not create private methods

[brson]

This suprised me:

impl foo {
  // This method is private
  priv fn bar() { }
}

priv impl foo {
  // This method is public even though the impl is private.
  fn bar() { }
}

priv impl doesn't do anything.

[catamorphism]

My understanding is that impls just shouldn't allow pub/priv on the entire impl, so #3985 supersedes this.

Reopen if I've misunderstood.