Add memory canaries and turn them on for unit tests

Add memory canaries and turn them on for unit tests

This will allow us to verify with resonable certainty
that the code is not writing past the end or beginning
of the memory allocation that it was given.

With canaries on, the canaried pointers returned by the
osi alloc functions are no longer passable directly to
free, since they must be offset to get the real allocation
pointers. This means all legacy code must be converted
over to the osi alloc functions before turning canaries
on in the wider codebase.