Add member function check_tokens();
Update make_root_tip();
Rename reroot_mappings_at => reroot_at
Allow non-root tokens to be dirty.

Cleaning a token can make its children dirty - that's the way its
supposed to work!
Only increase version if t1 is the root.
Fix share_and_clear( ).
Add assert.
Remove some uses of vm_absolute.
Remove check from set_reg_value( ).
Speed up by removing some checks.