Profiling data show that Allocation::operator= is hot (see the data
attached to the Phab review).
Reorder a few fields within Allocation to avoid implicit structure
padding and shrink the structure. This should make copies a bit cheaper.
Also, given that an Allocation contains a std::vector (by way of
DataBufferHeap), it's preferable to make it move-only instead of
permitting expensive copies. As an added benefit this allows us to have
a single Allocation constructor instead of two.