Separate flowR out into modules (properly this time)

[Ellpeck]

We attempted this once before, with separate repos, and it didn't go particularly well. We'd like for components like the CLI, the benchmarking, and the statistics to all be separate modules, but with some important points to keep in mind:

• they should be in the same repo for maintainability (we could potentially use changesets for release management)
• cross-dependencies should be local (rather than through npm) so that, for example, code changes to flowR's dataflow analysis can immediately be benchmarked without having to create a release
• flowR core's dependencies should be kept as minimal as possible, to avoid issues like Various flowR dependencies stop it from being webpack-able #757