[PATCH] improve performance of std::allocator::deallocate

[Pádraig Brady]

Attached is a simple patch which has been extensively tested within
Facebook,
and is enabled by default in our code base.

Passing the size to the allocator allows it to optimize deallocation,
and this was seen to significantly reduce the work required in jemalloc,
with about 40% reduction in CPU cycles in the free path.

Note jemalloc >= 5.1 is required to fix a bug with 0 sizes.

cheers,
Pádraig
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