A lot of the helpers in `helpers.ts` are heavily bound to NodeJS and at
the moment we're trying to make the `Connection` class be able to run in
multiple environments. Its only remaining Node dependency was its
reliance on `helpers.ts`, which it only needed for `assert`.
This is a useful change also because `helpers.ts` is quite large and
full of functions that do different things; I think we can name them
better and move them into modules with a specific purpose rather than a
generic `"helpers"` dumping ground.
Once this change lands `Connection` should be usable in the browser.
It conflicts with an inbuilt TypeScript `Request` type so can cause
confusion when in TS land. Note: `Response.ts` and `Worker.ts` also
suffer from this; PRs to rename them are incoming.
