REF: share PeriodArray.asfreq with Period.asfreq

[jbrockmendel]

Slightly faster for PeriodArray, no change for Period.

In [3]: pi = pd.period_range("2016-01-01", periods=100, freq="D")

In [4]: %timeit pi.asfreq("W")
43.5 µs ± 696 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each)  # <-- master
36.3 µs ± 1.06 µs per loop (mean ± std. dev. of 7 runs, 10000 loops each)  # <-- PR

In [5]: per = pi[0]

In [6]: %timeit per.asfreq("W")
10.5 µs ± 88.7 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)  #  <-- master
10.5 µs ± 160 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)  # <-- PR