Refactoring arrows

At the moment arrow commands re-use HsExpr for Proc expressions. But with Dan Winograd-Cort I concluded that the best thing by far would be to separate the data type of HsExpr from that of arrow commands. I think that would lead to a substantial tidy up.

There is also a nasty lurking bug in the type checking of commands; see line 290 of TcArrows. Here we call the unifier, but do not do anything with the coercion it returns. This is plainly wrong and will bite eventually. But I don't understand this code well enough to fix it.

In short, I am not confident of the arrows implementation at the moment.

Several tickets are blocked on this

• #5022 (closed) (which is marked as fixed but is a pretty tricky case)
• #5267
• #5777 (closed)
• #5609 (closed)
• #344 (closed)
• #4359 (closed) proposal for proc case and multi-branch if