Use -O3 in performance tests

[99991]

There are no optimization flags passed to the compiler in the makefile, but if one passes -O3 to g++ here, the compiler will remove all benchmark code because the result is not being used anywhere and the duration will be zero. I would suggest to add the -O3 flag to CCFLAGS and adjust the code to not be deleted by the optimizer.

[igushev]

The structure is sensitive and benefits a lot from using reserve(). I need to add automatic restructuring (any volunteers? :-) ) as it grows and shrinks.
Try same test, but use reserve() in advance.