[FEA] Implement applymap method for StringColumn

[tgrel]

Is your feature request related to a problem? Please describe.
Currently issuing the following code results in an exception:

import cudf
data = 'a b c d e f g'.split()
s = cudf.Series(data)
s.applymap(lambda x : x)

The complete traceback:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-13-8dbb669ac599> in <module>
      4 s = cudf.Series(data)
      5 
----> 6 s.applymap(lambda x : x)

/conda/envs/rapids/lib/python3.6/site-packages/cudf/dataframe/series.py in applymap(self, udf, out_dtype)
   1532             The mask and index are preserved.
   1533         """
-> 1534         res_col = self._column.applymap(udf, out_dtype=out_dtype)
   1535         return self._copy_construct(data=res_col)
   1536 

AttributeError: 'StringColumn' object has no attribute 'applymap'

For reference I'm using rapids in a docker image: nvcr.io/nvidia/rapidsai/rapidsai:cuda10.0-base-ubuntu16.04

print(cudf.__version__) results in 0.8.0+0.g8fa7bd3.dirty

Describe the solution you'd like
I'd like to be able to compute apply_map() for a string column similarly to apply in pandas.

Describe alternatives you've considered
N/A

Additional context
N/A

[GregoryKimball]

Let's close this in favor of #9639