Fix negative unary operation for boolean type

[galipremsagar]

Description

Negative unary op on boolean series is resulting in conversion to int type:

In [1]: import cudf

In [2]: s = cudf.Series([True, False])

In [3]: s
Out[3]: 
0     True
1    False
dtype: bool

In [4]: -s
Out[4]: 
0   -1
1    0
dtype: int8

In [5]: -s.to_pandas()
Out[5]: 
0    False
1     True
dtype: bool

The PR fixes the above issue by returning inversion of the boolean column instead of multiplying with -1.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[galipremsagar]

/merge