Count how many mappings are currently active and also with what access pattern.
Update the design doc with restrictions on the access patterns for nested
mappings.
Check if nested mappings obey the access mode restrictions of the design doc.
Add various unit tests to check the desired behaviour.
Require the memory implementations to implement a share operation. This allows
us to remove the fallback share implementation which uses a different allocator
implementation and complicates things too much.
Update design doc a bit.
