Change remote to count down 0, so fast path does not need a constant.

Use signed value so that branch does not depend on addition.

The fast path of remote_dealloc is sufficiently compact that it can be
inlined.

Turn the internal structure into tail calls, to improve fast path.
Should be no algorithmic changes.

Break lazy initialisation into two functions, so it is easier to codegen
fast paths.

Make the backup path a bit faster.  Only algorithmic change is to delay
checking for first allocation. Otherwise, should be unchanged.

The fisrt operation a new thread takes is special.  It results in
allocating an allocator, and swinging it into the TLS.  This makes
this a very special path, that is rarely tested.  This test generates
a lot of threads to cover the first alloc and dealloc operations.

Large alloc stats aren't necessarily balanced on a thread, this changes
to tracking individual pushs and pops, rather than the net effect
(with an unsigned value).

Each allocator has a bump ptr for each size class.  This is no longer
slab local.

Slabs that haven't been fully allocated no longer need to be in the DLL
for this sizeclass.

This change reduces the branching in the case of finding a new free
list. Using a non-empty cyclic list enables branch free add, and a
single branch in remove to detect the empty case.

Use needs initialisation as makes more sense for other scenarios.

This reverts commit a857b92.

The GlobalPlaceholder allocator is now a zero init block of memory.
This removes various issues for when things are initialised. It is made read-only
to we detect write to it on some platforms.