[WIP] Return events from TextEdit

[daboross]

This is still largely incomplete, inefficient, will break when used with certain configurable event transformers and has bugs with line_infos.

Still, it runs! The code demonstrates at least part of the design, and does compile.

Progress so far: I have added TextEvent, changed TextEdit to give Vec, and modified things enough to compile.

Things left to do:

• handle possible insertions during the deletion path, and deletions during the insertion path. event_transform can turn any event into any other event, so this will be necessary
  • we'll likely just want an "apply_event" function or closure to do this
• In general, rework code which could be made cleaner under the event design
• migrate TextBox to return events as well. It currently functions, though, so this could be done in a future PR
• add support for "side-extending" text boxes. I've got no current idea how to do this, but I'd like to be able to support username/password input boxes which allow the user to type a lot of text, and just scroll to the side when they do this. Might be a future PR.

Besides pure programming, there are a few things I'm not sure what to decide on:

• decide on whether to use smallstring::SmallString, String, or char for events. We're usually just typing one character at a time, but we might type more. I chose SmallString initially as a compromise, but we might want to do something different.

• decide on if support for "passing through opaque "data" events" from event transformers is a good idea

• decide on if event transformers themselves are a reasonable idea.

  I chose to add them because I wanted to uphold "multiple keypress/typing/clicking events should function the same whether they're in the same conrod 'frame' or not". If a TextEdit wrapper was choosing to disallow numbers, the "event transform" allows TextEdit to know the wrapper has done this when it's processing the next bit of input.

Replaces #964.