Add len() for Index & MultiIndex

[itholic]

an alias for size

>>> pidx = pd.Index(range(1000))
>>> kidx = ks.Index(range(1000))
>>> len(pidx)
1000
>>> len(kidx)
1000

for MultiIndex

>>> pidx = pd.MultiIndex.from_tuples([('a', 'x', 1), ('b', 'y', 2), ('c', 'z', 3)])
>>> kidx = ks.MultiIndex.from_tuples([('a', 'x', 1), ('b', 'y', 2), ('c', 'z', 3)])
>>> len(pidx)
3
>>> len(kidx)
3

[codecov-io]

Codecov Report

Merging #1161 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1161      +/-   ##
==========================================
+ Coverage   95.22%   95.22%   +<.01%     
==========================================
  Files          35       35              
  Lines        7059     7061       +2     
==========================================
+ Hits         6722     6724       +2     
  Misses        337      337

Impacted Files	Coverage Δ
databricks/koalas/indexes.py	96.66% <100%> (+0.02%)	⬆️

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