RangeSet: support negative ranges (#515)

[thiell]

Support negative ranges in RangeSet, with as little impact as possible to the normal (positive range) codepath. Zero padding is not available with negative ranges at this time for this reason.

In case of negative ranges, RangeSet will case the numbers to integer. Internal sorting then looks like this (eg. -5-6):
['-5', '-4', '-3', '-2', '-1', '0', '1', '2', '3', '4', '5', '6']

Apart from the internal sorting change, little modifications are needed in the parser and the folding method, so I guess it's worth it.

Usage examples:

>>> RangeSet(range(-5,7)) - RangeSet(range(1,3))
-5-0,3-6

>>> list((RangeSet(range(-5,7)) - RangeSet(range(1,3))).intiter())
[-5, -4, -3, -2, -1, 0, 3, 4, 5, 6]

>>> list((RangeSet(range(-5,7)) - RangeSet(range(1,3))).contiguous())
[-5-0, 3-6]

Hence, the following is now possible:

$ cluset -f foo[-10-10]
foo[-10-10]

$ cluset -f foo[-100--90/2]
foo[-100,-98,-96,-94,-92,-90]

Closes #515.

[degremont]

That looks great (negative value support is creating some crazy rangeset patterns :p )

Any concern on the perf impact, mostly for sorted?

[thiell]

Thanks for the quick review! No real concern here w.r.t. performance as the sorting method for positive ranges remains the same, on the same kind of tuples (len(x), x), although it's true that there is a if condition added to check for -. But after a quick test (in Python 2 only, btw we should port the benchmarks to Python 3 to be more relevant nowadays), there nothing really significant I can detect:

[sthiell@dev-centos7 bench.py2_neg-ranges]$ ./UnitBench.py compare master b191_515
 TESTNAME                 master   b191_515
 NodeSet15Add            0.08578    0.08793        3%
 NodeSet15Contains       0.09407    0.09806        4%
 NodeSet15Iter           0.00115    0.00136       18%
 NodeSet15Parse          0.07032    0.07151        2%
 NodeSet15Pickle         0.00579    0.00621        7%
 NodeSet15Remove         0.14756    0.15284        4%
 NodeSet15Str            0.00421    0.00440        4%
 NodeSet15Union          0.00019    0.00019       -3%
 NodeSet2DAdd            0.08004    0.07899       -1%
 NodeSet2DContains       0.39155    0.41084        5%
 NodeSet2DIter           0.13716    0.14571        6%
 NodeSet2DParse          0.40551    0.40850        1%
 NodeSet2DRemove         1.69130    2.02133       20%
 NodeSet2DStr            0.13921    0.15779       13%
 NodeSet2DUnion          0.15036    0.15690        4%
 NodeSet2DcPickle        0.05444    0.05821        7%
 NodeSetAdd              0.04538    0.04167       -8%
 NodeSetContains         0.04776    0.04288      -10%
 NodeSetIter             0.00109    0.00117        7%
 NodeSetParse            0.03697    0.03408       -8%
 NodeSetRemove           0.05286    0.04723      -11%
 NodeSetStr              0.00444    0.00416       -6%
 NodeSetUnion            0.00012    0.00010      -16%
 NodeSetcPickle          0.00582    0.00556       -4%
 RangeSetAdd             0.00179    0.00181        1%
 RangeSetContains        0.00158    0.00178       13%
 RangeSetIter            0.00073    0.00092       26%
 RangeSetParse           0.00763    0.00794        4%
 RangeSetRemove          0.00331    0.00326       -2%
 RangeSetStr             0.00420    0.00431        3%
 RangeSetUnion           0.00009    0.00009       -0%
 RangeSetcPickle         0.00538    0.00572        6%