Implement ResumableSink except for its core, resumeSink
Implement ++$$, completing Data.Conduit.Resumable
Add newResumable*

I was originally reluctant to do this, as it might lead to less
consistency in application code (you can also say `src $$+ return ()`,
which seems just as easy).  However, creating an initial ResumableSink
is less convenient because of the 'Either' return value in '+$$'.
Add ResumableSink tests

Taken from the conduit-resumablesink package by Andrew Miller
Alter indentation of '++$$' to match other operations