Add devel: GHC 7.8.1 rc1.

After a couple months of delays, it looks like GHC 7.8 is basically here.

Worth reading: http://www.well-typed.com/blog/85

As an aside: I've been critical of some complicated formulae in the past, so I thought I'd take the opportunity to say I think this one is done beautifully well. There's some very tricky stuff here, but because its all blocked together appropriately (and heredocs and assertions aren't scattered everywhere), it was trivial to add this. I don't know who get's the credit for that, but thanks; and props!