Scan naming in the new scanl module

[harendra-kumar]

Since postscan is the most common use case we can use scanl for postscan and use scanl0 for regular scan that emits the accumulator as well. scanl1 as usual will use the first element in the stream as accumulator.

[harendra-kumar]

Better, rename the standard scanl/scanl1 to prescanl/prescanl1 and rename postscanl/postscanl1 to scanl/scanl1. But this would require lot of changes.