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/os/env.c

@@ -561,14 +561,11 @@ void home_replace(buf_T *buf, char_u *src, char_u *dst, int dstlen, bool one)
     homedir_env = NULL;
 
   if (homedir_env != NULL && vim_strchr(homedir_env, '~') != NULL) {
-    int usedlen = 0;
-    int flen;
-    char_u  *fbuf = NULL;
-
-    flen = (int)STRLEN(homedir_env);
-    (void)modify_fname((char_u *)":p", &usedlen,
-        &homedir_env, &fbuf, &flen);
-    flen = (int)STRLEN(homedir_env);
+    size_t usedlen = 0;
+    size_t flen = STRLEN(homedir_env);
+    char_u *fbuf = NULL;
+    (void)modify_fname((char_u *)":p", &usedlen, &homedir_env, &fbuf, &flen);
+    flen = STRLEN(homedir_env);
     if (flen > 0 && vim_ispathsep(homedir_env[flen - 1]))
       /* Remove the trailing / that is added to a directory. */
       homedir_env[flen - 1] = NUL;