Make SeriesType, DataFrameType, and ScalarType hold the dtypes as well.

[ueshin]

After the return type inference, SeriesType, DataFrameType, and ScalarType hold the dtypes as well as Spark DataTypes.

[codecov-io]

Codecov Report

Merging #2123 (3ad837f) into master (ed0bd49) will increase coverage by 0.00%.
The diff coverage is 89.74%.

@@           Coverage Diff           @@
##           master    #2123   +/-   ##
=======================================
  Coverage   95.37%   95.37%           
=======================================
  Files          60       60           
  Lines       13587    13591    +4     
=======================================
+ Hits        12958    12962    +4     
  Misses        629      629           

Impacted Files	Coverage Δ
databricks/koalas/typedef/typehints.py	95.40% <86.36%> (+0.58%)	⬆️
databricks/koalas/groupby.py	94.31% <87.50%> (-0.12%)	⬇️
databricks/koalas/accessors.py	93.50% <100.00%> (+0.03%)	⬆️
databricks/koalas/frame.py	96.52% <100.00%> (+<0.01%)	⬆️
databricks/koalas/series.py	96.92% <100.00%> (ø)
databricks/koalas/utils.py	95.66% <0.00%> (-0.09%)	⬇️

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[xinrong-meng]

Looks great! Thank you!

[ueshin]

Thanks! merging.