First stage of unit tests for the page-table finished.

Test extension of the page-tables, and pruning; mapping addition and
deletion.  TODO:  fuzz tests.

Currently, the internal nodes (pgd entries) have fixed flags.  Only
the pte entries are configurable.
move the pgtbl/captbl/ertrie into the src/kernel/include directory
Merge branch 'ppos' of https://github.com/gparmer/Composite