Enable -Wconversion: normal.c.

Refactor summary:
- extern int opcount --> extern long opcount
- bool find_decl(..., int len, ...) --> bool find_decl(..., size_t len, ...)
  * int find_ident_under_cursor(...) --> size_t find_ident_under_cursor(...)
    - int find_ident_at_pos(...) --> size_t find_ident_at_pos(...)
    - int modify_fname(..., int *usedlen, ..., int *fnamelen) --> int modify_fname(..., size_t *usedlen, ..., size_t *fnamelen)
      * char_u *eval_vars(..., int *usedlen, ...) --> char_u *eval_vars(..., size_t *usedlen, ...)
        - int find_cmdline_var(..., int *usedlen) --> ssize_t find_cmdline_var(..., size_t *usedlen)
        - static char_u *repl_cmdline(..., int srclen, ...) --> static char_u *repl_cmdline(..., size_t srclen, ...)
    - bool get_visual_text(..., int *lenp) --> bool get_visual_text(..., size_t *lenp)
      * char_u *find_file_name_in_path(..., int len, ...) --> char_u *find_file_name_in_path(..., size_t len, ...)
        - static char_u *eval_includeexpr(..., int len) --> static char_u *eval_includeexpr(..., size_t len)
        - char_u *find_file_in_path(..., int len, ...) --> char_u *find_file_in_path(..., size_t len, ...)
          * char_u *find_file_in_path_option(..., int len, ...) --> char_u *find_file_in_path_option(..., size_t len, ...)
            - char_u *find_directory_in_path(..., int len, ...) --> char_u *find_directory_in_path(..., size_t len, ...)
      * int spell_move_to(...) --> size_t spell_move_to(...)
        - int spell_check(...) --> size_t spell_check(...)
        - static int spell_bad_len --> static size_t spell_bad_len
    - void find_pattern_in_path(..., int len, ...) --> void find_pattern_in_path(..., size_t len, ...)

Helped-by: Justin M. Keyes <justinkz@gmail.com>

src/nvim/screen.c

@@ -2465,7 +2465,7 @@ win_line (
     /* When spell checking a word we need to figure out the start of the
      * word and if it's badly spelled or not. */
     if (has_spell) {
-      int len;
+      size_t len;
       colnr_T linecol = (colnr_T)(ptr - line);
       hlf_T spell_hlf = HLF_COUNT;
 
@@ -2485,7 +2485,8 @@ win_line (
         word_end = (int)(spell_to_word_end(ptr, wp) - line + 1);
       } else {
         /* bad word found, use attributes until end of word */
-        word_end = wp->w_cursor.col + len + 1;
+        assert(len <= INT_MAX);
+        word_end = wp->w_cursor.col + (int)len + 1;
 
         /* Turn index into actual attributes. */
         if (spell_hlf != HLF_COUNT)
@@ -3252,8 +3253,9 @@ win_line (
             else
               p = prev_ptr;
             cap_col -= (int)(prev_ptr - line);
-            len = spell_check(wp, p, &spell_hlf, &cap_col,
-                nochange);
+            size_t tmplen = spell_check(wp, p, &spell_hlf, &cap_col, nochange);
+            assert(tmplen <= INT_MAX);
+            len = (int)tmplen;
             word_end = v + len;
 
             /* In Insert mode only highlight a word that