Convert a bunch of types and functions from String -> Text

[ChrisPenner]

Overview

Sorry, I know this is annoying and spans a big chunk of the codebase.

Switches a bunch of stuff from String -> Text;
We're still using String in a lot of low-level stuff, which is going to be inefficient both in memory and cpu when doing a bunch of text munging.

Specifically I wanted to switch AnnotatedText to actually use Text so I can use Text primitives when working with it (without needing to switch back and forth a bunch).

I'll be building some of the diffing improvements on top of this.

Implementation notes

• Switch AnnotatedText to use Text, then propagate that out to natural stopping points.
• I didn't push it all the way out, but went as far as I reasonably coul.

Test coverage

Existing tests