Unit tests for the captbl are in a good shape, and page-table code is rewritten.

Done:
- unit tests for captbl
- most functionality tested, but perhaps some edge cases are lurking
- rewrite of the page-table code, but still haven't tested

TODO:
- test the page-table code
- fuzz tests for the captbl code -- complicated by expand/shrink
- fuzz tests for the page-table code
- see if there is a way to modify the ertrie code to better accomodate some of the common patterns