BUG: .asfreq on resample on PeriodIndex/TimedeltaIndex are not
closes #12926

a tad hacky on this soln, as seems `DatetimeIndex` does lots of manipulations inside the binner to fix fencepost issues, while the other indexes (Period/Timedelta) don't seem to need this.

cc @MaximilianR created a structure to support common testing for resampling (the `Base` test class)
 Nice re the tests.

Tangential: I had a go deprecating the `how` keyword to make the Period stuff much simpler. I ran into issues with simple upsampling into super periods; I don't think that is possible without `DatetimeIndex` machinery atm. It's on my list though.
 yep - I think we can still use much of the DatetimeIndexResampler machinery - just return the PeriodIndex at the end (and of course handle special cases)
