Updates to Yesod and WAI

This brings WAI forward to use GHC 7.8, just like Yesod does now. It also introduces some performance enhancements to the Yesod implementation, and reduces the library dependencies in Yesod to get faster compile times (it originally used the yesod package, when in reality just yesod-core was necessary).

One possibly contentious change is: 062e4f9e531a554812f984fce5ca02293ae6fab7. @kazu-yamamoto and I discussed this, and weren't completely certain if the thread count passed in should be treated as a hard limit on the number of machine cores to use or not. This commit changes it so that the benchmarks will use exactly the number of threads specified, regardless of the number of cores actually available. The previous behavior was to use all cores reported available by the OS.