Introduce a `OnePastEnd` option for the pointer immediately after the
end of the allocation. This simplifies some of the logic in callers,
where they wants to say 'is base + length safe to use?'.
Also restructure some of the other logic somewhat.
The current pagemap assumes there is at least one level of indexing.
This commit introduces a new pagemap that is completely flat. This is
useful for 32bit, where there is no need to introduce the indexing
structure.
This pagemap is also considerably faster for 64bit platforms, but
does require a global allocation of 16MiB of the flat page map.
The design of Remote used the top bits of the allocator id to encode
the sizeclass of the deallocation. On 32bit, or on a platform that uses all the bits
we cannot use these bits for a sizeclass.
This commit uses the bottom bit of the allocator id (which is
guaranteed to be 0), to indicate if the object is the minimum
allocation size. If it is not the minimum allocation size the
subsequent byte is used to encode the sizeclass.
The code uses constexpr to decide which strategy to use.
