CLN: Series.asof

[bwillers]

As discussed in #10266 Series.asof is effectively equivalent to s.dropna().reindex(where, method='ffill'). The implementation of Series.asof uses Index.asof_locs, which is used no where else in the code, and is untested. So deprecating Series.asof might make sense.

Let's add the impls discussed (IOW, implement .asof in terms of reindexing internally). Further let's add tests for .asof_locs (though this maybe abel to tbe removed)

[jbrockmendel]

We have decent tests for Index.asof and DatetimeIndex.asof. Closing.