WIP: Page cache over a std::istream

The idea here is to reduce the amount of seeking and rereading of data
by putting a page cache over an input stream.  Initial naive testing
indicates that it might not help much for local files, but there's lots
more to do.

* Rudimentary page cache class for std::istream, without proper cache
  rejection so far (memory usage keeps growing until the whole file is
  cached!)

* Hook up to HCloudView in a pretty crap way for an initial test.