NodeBuffer append is this.type

[som-snytt]

Fixes #650

The signature should be like https://github.com/scala/scala/blob/v2.13.15/src/library/scala/collection/mutable/StringBuilder.scala#L142 so that linting knows it does not return a new value.

The generated code has the form

    private[this] val xml: scala.xml.Elem = new scala.xml.Elem(null, "xml", scala.xml.Null, scala.xml.TopScope, false, ({
      val $buf: scala.xml.NodeBuffer = new scala.xml.NodeBuffer();
      $buf.&+(new scala.xml.Elem(null, "elem", scala.xml.Null, scala.xml.TopScope, false));
      $buf
    }: _*));

[dubinsky]

I do not understand why this change does not trigger the compatibility checker like other attempts to make the types more precise do, but I am all for this change!
Let's just wait a couple of days before merging it to give the people an opportunity to speak up.

[SethTisue]

MiMa analyzes bytecode. (There are a few situations in which it also consults the Scala signature, but that's not in play here.) this.type erases to the same thing as the old type, so MiMa (rightly) doesn't see anything to object to.

[som-snytt]

I sympathize with the preceding question and answer. Compiler fictions FTW!