Switch search textbox to operate on graphemes instead of bytes

[itsjunetime]

Should fix #146.

The things this does:

1. I got rid of the word_index in the search state - it's not reasonable to keep that around and recompute it every time you enter a char just in case you want to use the control+{left,right} keys to move, so we just find the word boundaries on the fly with the unicode_segmentation crate as well.
2. We now consider the 'first writeable column' in the search state to be 0, instead of 1 (even though 1 is technically the column number the which this string will be written in the terminal), since that just makes everything easier to reason about. We just adjust for this when we actually display the string.
3. Simplified some test helpers, moved some things around to make it easier to process and more resilient to future refactors, etc
4. Added more tests

[itsjunetime]

Oh yeah: the basic_dynamic_paging test messes with the terminal state and erroneously exits with a zero exit code in such a way that CI is getting both false positives and false negatives. So I need to work on that before this can function. See rust-lang/cargo#16558