Add packet stream parser

[brianc]

This is mostly a work in progress look at some medium-term performance & maintainability improvements I want to make, by re-writing the parser, adding typescript, and starting to use this module "from the inside out" to find out where the perf bottlenecks are. I've identified a pretty significant (2x!!) performance improvement on some experimental code by removing some intermediate event emitters and streamlining the parsing code quite a bit. I'm not sure there's a way to land it in a non-disruptive way so I'm still investigating, but I want to land some of this work-in-progress stuff. It's in its own module right now so it shouldn't disturb the main library at all.