Add a benchmark for the rxx_allocator.

This comes in two flavors:

1) A single pool and multiple allocations of 64 bytes. Somewhat
representative for the creation of the big AST tree.

2) Multiple pools with just a few allocations, as done by the
ExpressionParser.

Current results on my machine in RelWithDebInfo mode:

RESULT : TestPool::benchManyPools():
0.039 msecs per iteration (total: 80, iterations: 2048)
RESULT : TestPool::benchManyAllocations():
0.000068 msecs per iteration (total: 72, iterations: 1048576)