Implement benchmarks

[jack-pappas]

It would be good to have some benchmarks for fsharplex/fsharpyacc to quantify their performance; this would be particularly useful when implementing performance improvements, so we can see the performance before and after the improvements (to make sure they're really improvements!)

The benchmarks will need to cover both the run-times of fsharplex/fsharpyacc themselves (i.e., how long they take to compile a specification) and the run-times of the generated F# code (by running the generated lexers/parsers against some input file(s)).