better cursor search

[conradludgate]

I noticed we were using char indices for cursor support, but we can use utf8 byte indices for better performance. Refactored into a Cursor type with proper tests so we don't accidentally break it in future

[conradludgate]

Sorry, those last 2 commits are pretty chunky. I'd suggest reviewing by commit.

The last 2 commits are

• move methods to state - where I changed methods from taking app: &mut State and instead made them &mut self methods within impl State.
• refactor search modules - where I moved the interactive search and the Events/Cursor type as well into the search directory.

[conradludgate]

I can split the PR if you want :P