Updates for base-4.7.0 (GHC 7.8)

This adds the new `withMVarMasked` and `tryReadMVar` functions.  They seem to work in GHCI, but I haven't tested them extensively.

This also changes which modules are exported for GHC 7.6 - as `Control.Concurrent.QSem.Lifted` and `Control.Concurrent.QSemN.Lifted` weren't being exported - even though the corresponding modules haven't been removed from any published version of base.

I've tested compilation with GHC 7.4 and GHC 7.8.
Ah, interesting!  Not only are these modules still present in base-4.7, they're no longer deprecated, and the documentation has been improved.