Printer: add an empty line between bindings in Rec STG binding groups

Before:

    Rec {
    x2_r10T :: Lib.Bar
    [GblId, Unf=OtherCon []] =
        CCS_DONT_CARE Lib.Bar! [x3_r10U];
    x3_r10U :: Lib.Foo
    [GblId, Unf=OtherCon []] =
        CCS_DONT_CARE Lib.Foo! [x1_r10p x2_r10T];
    end Rec }

After:

    Rec {
    x2_r10T :: Lib.Bar
    [GblId, Unf=OtherCon []] =
        CCS_DONT_CARE Lib.Bar! [x3_r10U];

    x3_r10U :: Lib.Foo
    [GblId, Unf=OtherCon []] =
        CCS_DONT_CARE Lib.Foo! [x1_r10p x2_r10T];
    end Rec }

compiler/stgSyn/StgSyn.hs

@@ -732,7 +732,7 @@ pprGenStgBinding (StgNonRec bndr rhs)
 
 pprGenStgBinding (StgRec pairs)
   = vcat [ text "Rec {"
-         , vcat (map ppr_bind pairs)
+         , vcat (intersperse blankLine (map ppr_bind pairs))
          , text "end Rec }" ]
   where
     ppr_bind (bndr, expr)