Fix benchmarks and don't cheat in them

simplewiki benchmark was worthless because it used a superset of test
data as corpus data. That's cheating. This commit fixes it.

Additionally,

  - add a simplewiki benchmark for 30B+ strings (for comparison)
  - instead of sort use Ruby for shuffling (portability)
  - use IO.popen and IO#write to provide input in benchmark.rb (ditto)