[BUG] Incorrect slice on struct column.

[shaneding]

Describe the bug
This bug is two fold. Firstly, cpp_copying.slice() generates an incorrect slice of the underlying struct column. Furthermore, the fields of the column after slicing is also incorrect (needs to be correctly repopulated).

Steps/Code to reproduce bug

In [15]: import cudf
In [16]: sr = cudf.Series([{"a": "Hello world"}, {"a": "String 2"}, {"a": "String 3"}])
In [17]: sr[1:]
Out[17]: 
1    {'0': 'Hello world'}
2       {'0': 'String 2'}
dtype: struct
In [18]: 

Expected behavior

1       {'a': 'String 2'}
2       {'a': 'String 3'}
dtype: struct