Modify navigation and history management.
- historyProxy, bfScheduler -> historyManager, scheduler
    + Different methods and usage
    + External code only uses historyManager
    + bufferPopState() not needed any more
- HFrameset#render() split into two phases
    + prepare() merges frameset <html>, <head> and is called by load()
    + render() now only merges frameset <body>
    + mergeHead() doesn't need to remove duplicate <script> - unused anyway
- HFrame#render() becomes #navigate() which is split into phases
    + fetch(src)
    + render(doc)
    + insert(result)
- HFramesetDefinition options.getTarget becomes options.lookup
- framer#start() is no longer wholly enclosed by scheduler.now()
    Just frameset.render() is enclosed (and indirectly via historyManager).
- changeset-lookup during startup is moved from HFrameset#render() to
    framer.start() and bookended by HFrameset#prepare() and #render()