Making ReifyTimestampsAndWindowsFn public

[Wesley Tanaka]

GatherAllPanes.ReifyTimestampsAndWindowsFn looks useful for giving 
MapElements, Filter, et al access to PaneInfo and BoundedWindow. Is 
there a reason why that functionality shouldn't be made into a public 
PTransform?  I filed https://issues.apache.org/jira/browse/BEAM-3035 
which can be resolved invalid if this is a bad idea.

More generally, it seems like ValueInSingleWindow is hardly used across 
the API.  Is there a reason to avoid it, either in the API or in user 
code or both?

--
Wesley Tanaka
https://wtanaka.com/