Add an abstraction for keeping track of virtual memory areas

Currently, the paging code in the guest can update page tables, but doesn't have any way to keep metadata about mappings. This makes doing things like implementing mprotect() on a region of virtual address space that isn't backed by a physical mapping quite difficult, which is a problem for wasmtime. We should instead have some area datastructure that can keep around metadata, allow registering a handler for page faults on a particular area, etc.