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/misc2.c

@@ -382,8 +382,8 @@ int vim_chdir(char_u *new_dir)
   char_u      *dir_name;
   int r;
 
-  dir_name = find_directory_in_path(new_dir, (int)STRLEN(new_dir),
-      FNAME_MESS, curbuf->b_ffname);
+  dir_name = find_directory_in_path(new_dir, STRLEN(new_dir),
+                                    FNAME_MESS, curbuf->b_ffname);
   if (dir_name == NULL)
     return -1;
   r = os_chdir((char *)dir_name);