The 'arena' allocator does not take memory alignment into account, which
can result in crashes at -O3 with gcc 7.5 (presumably because of SSE
instructions).
This accounts for alignment issues, fixing the issue.
Also do various cleanup in lib/allocators.
Reported by: Bellaz89, Freagarach
Comments by: vladislavbelov
Differential Revision: https://code.wildfiregames.com/D3181
This was SVN commit r24517.
