Constants and Message Types

[jolheiser]

Addresses #413

This PR also moves message types to an enum-style setup, which just makes it a bit easier to keep consistent rather than "free-form" strings.

[schollz]

wonderful! thank you so much for this :) :)

[maximbaz]

FYI one of the tests in tcp_test.go fails because it tries to modify a now constant 🙂

[jolheiser]

Very odd. I'm almost positive all the tests passed when I ran them locally.
I'll send in a fix ASAP if no one else has gotten to it.

EDIT: My bad! I did indeed introduce a test failure. I've sent a PR to fix it.
At the very least it shouldn't have caused any broken releases, just the failing test.