character_delete_left

 This backspaces, deleting the character before the cursor.

 If we are at the beginning of a line it attempts to join the current line
 with the previous line. A text line may be joined with the line before, but
 a fold line may not.

 The character is not lost forever, as it is put on the deleted item stack
 maintained by the editor. Items on this stack may be recovered using
 other_undelete and other_put.