Problem that happens when deleting characters in insert mode. #2

Pressing backspace in insert mode can crash the program at src/editor/editing.rs:46: cannot remove a char from the end of a string

To reproduce it:

1. Go to an empty line.
2. Press a and type aaa.
3. Press backspace.

The causes are:

1. insert_forward (a) increments row without checking the line length, so the cursor ends up past the end of the line.
2. remove_char then can't find the character and calls line.remove(len), which panics.
3. move_tonext (w) has the same problem. It also mixes characters and bytes (line.len()).

To fix it:

1. Clamp the cursor in insert_forward with (buffer.row + 1).min(len).
2. In remove_char, return early when the character isn't found.
3. In move_tonext, use chars().count() instead of len().