1.  When desugaring a [d| ... |], do not gensym the method names of
    an instance declaration, else we get very confused!

2.  We don't need to make fresh Names in DsMeta.mkGenSym, because
    the existing Name will do, I believe.  We'll do the genSym when
    we *run* the splice.  (This stuff makes my head hurt.)

    Assuming this is so, we can simplify the mkGenSym stuff still more.