Add support for 'whichwrap=[,]'

- Add new concept of MoveCaretWithArrow to complement MoveCaret so
  as not to interfere with non-keybinding uses of MoveCaret.

- Arrange for <Left> and <Right> in insert mode to use the new API.

- Implement caret moving with or without wrapping according to
  'whichwrap' settings.

- Add tests for moving the caret in insert mode near line breaks with
  various values of 'whichwrap'.