Improve Benchmark constructor and instance variables

[camillobruni]

• 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

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[danleh]

LGTM with nit and questions around async/isAsync.