Support arrays of BigInt, Range, Result, and Unit in entry point options

[bamarsha]

• Refactor the argument parsers so they support both single-valued and many-valued arguments (arrays). This was done by primarily supporting many-valued arguments (the more general case), and then calling Single() when the argument is single-valued.
• Pass the argument-parsing functions directly to the Option<T> constructors instead of creating Argument<T> instances and setting the Option<T>.Argument property. These should be functionally equivalent, it is just recommended to do it this way in System.CommandLine.
• Various cleanup:
  • Split EntryPointDriver.cs into three files.
  • Rename Result<T> to Validation<T> to avoid confusion with Q# Result.