This patch implements monad comprehensions, Trac #4370.
Thanks to Nils Schweinsberg for doing most of the heavy lifting.
I did quite a lot of related refactoring as well. Notably:
* Combined TransformStmt and GroupStmt into a single
constructor TransStmt; they share a lot of code.
I also made TransStmt into a record; it has a lot of fields.
* Remove the "result expression" field of HsDo, and instead
implement LastStmt, which is expected to be at the end
of a list of Stmts
* Generalise and tidy up the typechecking of monad comprehensions
* Do-notation in arrows is marked with HsStmtContext = ArrowExpr
* tcMDoStmt (which was only used for arrows) is moved
to TcArrows, and renamed tcArrDoStmt
* Improved documentation in the user manual
* Lots of other minor changes