Make the Session glue monomorphic

--HG--
extra : amend_source : f5e15d5909f971af7223de352e83f658ea63e961
Flesh out cache implementation a bit
Add /cache support to httpbin
Added a way to set Network.HTTP.Client.Request.checkStatus

Settable via an optional function that Wreq users can set through
Network.Wreq.Options.checkStatus
Forgot to export this from the top level. Fixed.
Style and doc fixes as requested by bos
Missed a quote around 'Nothing'
Rebase with bos/wreq