REF: use _get_string_slice in PeriodIndex.get_value
Most of _get_string_slice is duplicated inside PeriodIndex.get_value, which likely explains why get_string_slice has almost no test [coverage](https://codecov.io/gh/pandas-dev/pandas/src/master/pandas/core/indexes/period.py#L715).  This also brings PeriodIndex.get_value into closer alignment with DatetimeIndex.get_value, so we may be able to share code a few steps from here.

