Global types are defined differently for `.ts` files than for `.js`
files, leading to some confusion for past refactors.
This commit defines them correctly.
I'm also considering making certain global types only available under
namespaces, but I don't want to do that to `User` or `Room`, so for
now, there are no changes there, besides putting streams in the
`Streams` namespace (so `WriteStream` is now `Streams.WriteStream`).
`import =` and `export =` are really only intended for backwards
compatibility with CommonJS. While I really don't like the new module
system TC39 has designed for us, it's what we should be using, and
consistency is important.