Enable -Wconversion: ui.c.

Refactoring summary:
- ui_write(): len: int --> size_t
    * parse_abstract_ui_codes(): len: int --> size_t
    * string_convert(): lenp: int * --> size_t *
        - string_convert_ext(): lenp      : int * --> size_t *
                                unconvlenp: int * --> size_t *
            * utf8len_tab_zero: char[] --> uint8_t[]
            * iconv_string(): slen      : int --> size_t
                              unconvlenp: int * --> size_t *
                              resultlenp: int * --> size_t *
        - mch_print_text_out(): len: int --> size_t
    * out_pos: int --> size_t

src/nvim/ex_cmds.c

@@ -5257,8 +5257,7 @@ void fix_help_buffer(void)
                 else {
                   /* Do the conversion.  If it fails
                    * use the unconverted text. */
-                  cp = string_convert(&vc, IObuff,
-                      NULL);
+                  cp = string_convert(&vc, IObuff, NULL);
                   if (cp == NULL)
                     cp = IObuff;
                 }