PERF: significantly improve performance of MultiIndex.shape

[qwhelan]

MultiIndex.shape is currently extremely slow as it triggers the creation of ._values, which can be quite expensive for datetime levels. The one mitigating factor is that this result is cached and thus making ._values.shape near-instant on subsequent calls, but also hard to catch in asv benchmarks; this commit adds a suite dedicated to measuring such cached properties on Index objects.

asv results show a ~400,000x speedup for a relatively straightforward case:

       before           after         ratio
     [269d3681]       [d205acf6]
     <master>       <shape>   
-      3.52±0.02s       8.33±0.2μs     0.00  index_cached_properties.MultiIndexCached.time_shape

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[qwhelan]

Updated asv results show moving into Index benefits a few other classes significantly as well:

       before           after         ratio
     [a4c19e7a]       [3c946017]
     <unsorted_cats~1>       <shape>   
-     2.59±0.07μs      2.16±0.06μs     0.83  index_cached_properties.IndexCache.time_shape('PeriodIndex')
-     2.74±0.09μs       2.26±0.1μs     0.83  index_cached_properties.IndexCache.time_shape('DatetimeIndex')
-      5.06±0.2μs       3.57±0.2μs     0.70  index_cached_properties.IndexCache.time_shape('UInt64Index')
-      5.80±0.4μs       3.70±0.3μs     0.64  index_cached_properties.IndexCache.time_shape('Float64Index')
-      6.40±0.4μs       4.08±0.3μs     0.64  index_cached_properties.IndexCache.time_shape('TimedeltaIndex')
-      6.80±0.3μs       3.88±0.2μs     0.57  index_cached_properties.IndexCache.time_shape('IntervalIndex')
-        65.2±1μs         903±20ns     0.01  index_cached_properties.IndexCache.time_shape('Int64Index')
-      65.1±0.9μs         892±10ns     0.01  index_cached_properties.IndexCache.time_shape('RangeIndex')
-         214±2ms       4.45±0.2μs     0.00  index_cached_properties.IndexCache.time_shape('MultiIndex')