Type check the code using Sorbet

[mattmccutchen]

I've been working on statically type checking the Braid code using Sorbet. It's overdue that I file an issue to provide a central place for tracking progress and for any needed general discussion. PRs so far:

• Set up type checking using Sorbet and annotate parts of the code. #105
• Get most files in lib to typed: true + other small changes. #116
• Type-check most of operations.rb and fix handling of spaces. #119
• Finish first pass of type annotations for lib + other small changes. #123

The remaining piece is type checking the tests. I've started working on it and it appears that typed: true is feasible and adds significant value, while typed: strict is much more trouble than it's worth. When that's done, I think I'll finally be ready to resume nontrivial ordinary development.