Turn anonymous function bodies into regular defs

As one tiny step towards method handles, this change moves the body
of an anonymous function into a fresh ordinary def. Then an
anonymous subclass of FunctionN gets an apply method that calls that
new def.

Transform a function node (x_1,...,x_n) => body of type
   FunctionN[T_1, .., T_N, R]

into

{
  final def anonfun(x_1: T_1, ..., x_N: T_n): R = body
  class $anon() extends AbstractFunctionN[T_1, .., T_N, R]
      with Serializable {
    def apply(x_1: T_1, ..., x_N: T_n): R = anonfun(x_1, x_2, ..., x_N)
  }
  new $anon()
}
If we're in a constructor then just inline the function body into
the anonymous class's apply method.