* Add concept of natural alignment to tests.
snmalloc naturally aligns blocks very heavily, so that
the largest power-of-two in the rounded size is the alignment.
This checks that in the test, and provides a method for
finding the natural alignment of a block.
* Improve USE_MALLOC to provide alignment
snmalloc provides a lot of alginment guarantees. This ensures that when
we pass through to the system allocator we still get those alignment
guarantees.
The commit also fixes the tests to work with USE_MALLOC, and builds a
set of unit tests for ctest to check behaviour.
Made the API so that get always returns an initialised Alloc*. Added
new fast path that doesn't perform checking, but can lead to very slow
behaviour if called and reused.
By caching the result of the first call to ThreadAlloc::get(), we were
always hitting a code path that should be hit once per thread in normal
operation.
