New alternative argument parser: optparse-applicative

[elegios]

This PR introduces a new library for parsing command line flags that's a bit more flexible in how flags are specified, and importantly the ability to specify mutually exclusive flags.

The key idea of the interface is to have a central type OptParser a that represents a set of options that can be parsed from a commandline to produce a value of type a. This type is a functor (via optMap), an applicative (via optPure and optApply), and supports alternatives (via optOr), leading to a very composable interface. It's mildly more annoying to use in MCore than in Haskell because we don't have infix operators or ApplicativeDo, but hopefully the composability wins are worth it anyway.

The library is inspired by https://hackage.haskell.org/package/optparse-applicative and has a similar interface. Currently missing major things (that I think would be nice to have) relative to that library:

• The ability to specify repeatable flags (e.g., -v in some programs, or --debug-phase in mi).
• Subcommands (e.g., eval and compile for mi).
• Automatic generation of auto-complete interfaces.

Things we can do that are pretty neat:

• Build a nested value representing options without repeatedly traversing the same structure or handling partially constructed values.
• Mutually exclusive (or optional) sets of flags.
• Auto-generated --help text and flag.