Add basic extension dtypes support.

[ueshin]

Adds basic extension dtypes support.

The following types are supported if the underlying pandas supports them:

• pandas >= 0.24
  • Int8Dtype
  • Int16Dtype
  • Int32Dtype
  • Int64Dtype
• pandas >= 1.0
  • BooleanDtype
  • StringDtype
• pandas >= 1.2
  • Float32Dtype
  • Float64Dtype

Internally, index_dtypes and data_dtypes are introduced in InternalFrame.

>>> kdf = ks.DataFrame({'a': [1,2,None,3], 'b': [4,5,6,None]}).astype({'a': 'Int32', 'b': 'Int64'})
>>> kdf
      a     b
0     1     4
1     2     5
2  <NA>     6
3     3  <NA>
>>> kdf.dtypes
a    Int32
b    Int64
dtype: object

>>> kdf._internal.index_dtypes
[dtype('int64')]
>>> kdf._internal.data_dtypes
[Int32Dtype(), Int64Dtype()]

Currently binary operations and type casting are supported:

>>> kdf.a + kdf.b
0       5
1       7
2    <NA>
3    <NA>
dtype: Int64
>>> kdf + kdf
      a     b
0     2     8
1     4    10
2  <NA>    12
3     6  <NA>
>>> kdf.a.astype('Float64')
0     1.0
1     2.0
2    <NA>
3     3.0
Name: a, dtype: Float64

Resolves #2009.

[codecov-io]

Codecov Report

Merging #2039 (bb766a4) into master (dcf5275) will decrease coverage by 5.27%.
The diff coverage is 69.96%.

@@            Coverage Diff             @@
##           master    #2039      +/-   ##
==========================================
- Coverage   94.71%   89.43%   -5.28%     
==========================================
  Files          54       54              
  Lines       11503    11566      +63     
==========================================
- Hits        10895    10344     -551     
- Misses        608     1222     +614     

Impacted Files	Coverage Δ
databricks/koalas/namespace.py	78.49% <ø> (-5.92%)	⬇️
databricks/koalas/strings.py	82.35% <ø> (ø)
databricks/koalas/typedef/typehints.py	65.92% <25.71%> (-27.89%)	⬇️
databricks/koalas/base.py	93.46% <75.00%> (-3.83%)	⬇️
databricks/koalas/indexing.py	92.60% <91.66%> (-0.02%)	⬇️
databricks/koalas/internal.py	94.07% <94.33%> (-2.03%)	⬇️
databricks/koalas/frame.py	93.43% <100.00%> (-3.15%)	⬇️
databricks/koalas/indexes/base.py	97.23% <100.00%> (-0.21%)	⬇️
databricks/koalas/indexes/multi.py	91.45% <100.00%> (-4.35%)	⬇️
databricks/koalas/series.py	95.64% <100.00%> (-1.15%)	⬇️
... and 33 more

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[ueshin]

I think it's ready to review.
I'll address the boolean operations in a separate PR because "boolean" type has the semantic to be SQL-ish whereas "bool" is Python-ish.

[ueshin]

The DataFrame operations with different anchors seem not working now. I'll address it soon, but you can still start reviewing this PR.

[HyukjinKwon]

Haven't taken a close look but I guess it's good to go.

[ueshin]

Thanks! Let me merge this for now. Please feel free to leave comments if any.