refactor: replace char_u variables and functions with char

Work on https://github.com/neovim/neovim/issues/459

src/nvim/ex_eval.h

@@ -62,7 +62,7 @@ struct vim_exception {
   except_type_T type;                   // exception type
   char *value;           // exception value
   struct msglist *messages;        // message(s) causing error exception
-  char_u *throw_name;      // name of the throw point
+  char *throw_name;      // name of the throw point
   linenr_T throw_lnum;                  // line number of the throw point
   except_T *caught;          // next exception on the caught stack
 };