Make mirroring less cumbersome

[TiarkRompf]

Add a default case for mirrorDef using reflection.

Evaluate whether mirror can be made a method on Def (or a subclass). This would allow writing:

case class MyNode(x: Exp[A]) extends MirrorDef {
  def mirror(t: Transformer) => myop(t(x)))
}

def myop(x: Exp[A]) = MyNode(x)

Or:

case class MyNode(x: Exp[A]) extends MirrorDef(t => myop(t(x)))

def myop(x: Exp[A]) = MyNode(x)

Which may be simpler than the current mirror overrides.

[TiarkRompf]

Note: in the second version extends MirrorDef(t => myop(t(x))) we'd need to figure out how to pass the implicit SourceContext. Is there a better way than making it part of the Def instance?