Proposal: Add concatMapM function (#2042)

[Roman Leshchinskiy]

Bulat Ziganshin wrote:
> 
> the right way to deal with "modifiers" is to attach them to the Handle
> itself like this:
> 
> f <- openFile "name" >>= withLocking >>= withEncoding utf8

IMO, global state is never "the right way" if it can be avoided. It will 
always lead to problems. Especially in a functional language like Haskell.

Roman