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 }