Convert `benchmarks.rb` into a `benchmarks/` directory containing micro-benchmarks

[postmodern]

Convert the benchmarks.rb script into a benchmarks/ directory containing small Ruby benchmarks for various features of the Wordlist library (ex: enumerating a single wordlist, filter out duplicates, enumerating two concatenated wordlists, etc). This will make it easier to re-run benchmarks for a specific feature of the wordlists library.