Page Table Visitors

We previously had some discussion of using some sort of Visitor pattern to visit and update page tables. I got some time to play around with it a bit today. I think it is promising, but there are some unresolved API questions and some issues that I don't know how to deal with.

Unresolved Problem:

• The visitor doesn't know where in virtual memory a page table is. We need some generic and ergonomic way to figure out this information for Visitors to really work. For now, I have left it abstracted as the Visit::get_page method, which is implemented by each Visitor as needed.

Unresolved questions:

• Should we have visit_huge_page<S: Size>(...)? Is this too fine-grained?

Here is a rough prototype. It doesn't work or compile, but it should give a rough idea: https://github.com/rust-osdev/x86_64/compare/master...mark-i-m:visitor?expand=1

cc #32 #96 #112 #113 #114

Start by reviewing the linked visitor prototype comparison and the current Visit::get_page abstraction, then read the related issues #32, #96, #112, #113, and #114. Done means the unresolved page-table location and huge-page visitation API questions have a decided design and a compiling implementation.