New features

• Added Attributes::GP bit for BTI guarded pages.

Breaking changes

• zerocopy feature has been removed. PageTable::write_to is provided instead.
• IdMap::activate now returns the previous TTBR value rather than storing it, and
  IdMap::deactivate takes the TTBR value to restore as a parameter. IdMap::mark_active no longer
  takes a previous TTBR value parameter. The same applies to the equivalent methods on LinearMap.
• Renamed Mapping::activate_raw to activate, and added previous TTBR value parameter to
  Mapping::deactivate.
• A page table may be activated multiple times (e.g. on multiple cores) and will keep track of how
  many times it has been activated. It will only be considered inactive once it has been deactivated
  the same number of times.
• MapError::PteUpdateFault now contains a usize rather than a Descriptor.
• Descriptor no longer implements Copy, Clone, Default, PartialEq or Eq, as it now
  contains an AtomicUsize rather than just a usize. Various methods on Descriptor now take
  &self rather than self.