Breaking changes

• Added support for EL2 and EL3 page tables. This requires a new parameter to IdMap::new,
  LinearMap::new, Mapping::new and RootTable::new.
• Attributes::EXECUTE_NEVER renamed to Attributes::UXN.
• Attributes::DEVICE_NGNRE and NORMAL have been removed in favour of ATTRIBUTE_INDEX_*,
  OUTER_SHAREABLE and INNER_SHAREABLE, to avoid making assumptions about how the MAIR registers
  are programmed.

New features

• Added root_address, mark_active and mark_inactive methods to IdMap, LinearMap and
  Mapping. These may be used to activate and deactivate the page table manually rather than
  calling activate and deactivate.
• Added NS and PXN bits to Attributes.

Bug fixes

• When an invalid descriptor is split into a table, the table descriptors aren't set unless to
  non-zero values unless the original descriptor was.

Other changes

• Attributes::ACCESSED is no longer automatically set on all new mappings. To maintain existing
  behaviour you should explicitly set Attributes::ACCESSED whenever calling map_range for a
  valid mapping.