Reland: Improve Benchmark constructor and instance variables #255 (#263)

- Use plan as options bag for constructor arguments
- Prefix "protected" instance variables with "_"
- Rename Benchmark.preload to Benchmark._preloadEntries
- Move iteration and worstCase count processing to the Benchmark class
- Add files and preloadEntries getters so we can override it in GroupedBenchmark
- Rename plan.arguments to plan.args due to "argument" being a reserved word
- Rename WasmLegacyBenchmark to AsyncWasmLegacyBenchmark

Author: camillobruni