This is achieved by bumping `end_index` by a large enough amount so that
a suitably aligned region of memory can always be provided. The potential
wasted space this creates is then recovered by a single cmpxchg. This is
always successful for single-threaded arenas which means that this version
still behaves exactly the same as the old single-threaded implementation
when only being accessed by one thread at a time. It can however fail when
another thread bumps `end_index` in the meantime. The observerd failure
rates under extreme load are:

2 Threads: 4-5%
3 Threads: 13-15%
4 Threads: 15-17%
5 Threads: 17-18%
6 Threads: 19-20%
7 Threads: 18-21%

This version offers ~25% faster performance under extreme load from 7 threads,
with diminishing speedups for less threads. The performance for 1 and 2
threads is nearly identical.