This commit adds just the subsite itself. The subsite
works by running a list of generaters at compile time.
The entries produced by the generators are converted into
wai-app-static.WaiAppStatic.Storage.Embedded entries. Also,
addStaticContent is supported via an IORef. When a widget
produces static content (css, javascript), it is stuck into
the IORef inside the embedded static subsite. The embedded
static subsite will then serve it from the IORef, properly
using a 304 response if the client already has the content.
