Less repetitive mmap/unmap/mprotect code

Rewriten the mmap/unmap/mprotect to be much less repetitive.
There is a new "page_range_operation" class, from which the classes
"populate", "unpopulate", and "protect" classes derive to implement
mmap, munmap and mprotect respectively. The code is now much shorter,
less repetitive, clearer (I hope), and also better conforming to the
new coding conventions.

Note that linear_map is still separate, and page_range_operation keeps
its old algorithm (of starting at the root again for each page). Now
that we have this clean OO structure, it will be easier to change
this algorithm to be similar to linear_map's.