patch 8.2.2318: Vim9: string and list index work differently

Problem:    Vim9: string and list index work differently.
Solution:   Make string index work like list index. (closes #7643)

src/eval.c

@@ -5548,97 +5548,6 @@ eval_isdictc(int c)
     return ASCII_ISALNUM(c) || c == '_';
 }
 
-/*
- * Return the character "str[index]" where "index" is the character index.  If
- * "index" is out of range NULL is returned.
- */
-    char_u *
-char_from_string(char_u *str, varnumber_T index)
-{
-    size_t	    nbyte = 0;
-    varnumber_T	    nchar = index;
-    size_t	    slen;
-
-    if (str == NULL || index < 0)
-	return NULL;
-    slen = STRLEN(str);
-    while (nchar > 0 && nbyte < slen)
-    {
-	nbyte += MB_CPTR2LEN(str + nbyte);
-	--nchar;
-    }
-    if (nbyte >= slen)
-	return NULL;
-    return vim_strnsave(str + nbyte, MB_CPTR2LEN(str + nbyte));
-}
-
-/*
- * Get the byte index for character index "idx" in string "str" with length
- * "str_len".
- * If going over the end return "str_len".
- * If "idx" is negative count from the end, -1 is the last character.
- * When going over the start return -1.
- */
-    static long
-char_idx2byte(char_u *str, size_t str_len, varnumber_T idx)
-{
-    varnumber_T nchar = idx;
-    size_t	nbyte = 0;
-
-    if (nchar >= 0)
-    {
-	while (nchar > 0 && nbyte < str_len)
-	{
-	    nbyte += MB_CPTR2LEN(str + nbyte);
-	    --nchar;
-	}
-    }
-    else
-    {
-	nbyte = str_len;
-	while (nchar < 0 && nbyte > 0)
-	{
-	    --nbyte;
-	    nbyte -= mb_head_off(str, str + nbyte);
-	    ++nchar;
-	}
-	if (nchar < 0)
-	    return -1;
-    }
-    return (long)nbyte;
-}
-
-/*
- * Return the slice "str[first:last]" using character indexes.
- * Return NULL when the result is empty.
- */
-    char_u *
-string_slice(char_u *str, varnumber_T first, varnumber_T last)
-{
-    long	start_byte, end_byte;
-    size_t	slen;
-
-    if (str == NULL)
-	return NULL;
-    slen = STRLEN(str);
-    start_byte = char_idx2byte(str, slen, first);
-    if (start_byte < 0)
-	start_byte = 0; // first index very negative: use zero
-    if (last == -1)
-	end_byte = (long)slen;
-    else
-    {
-	end_byte = char_idx2byte(str, slen, last);
-	if (end_byte >= 0 && end_byte < (long)slen)
-	    // end index is inclusive
-	    end_byte += MB_CPTR2LEN(str + end_byte);
-    }
-
-    if (start_byte >= (long)slen || end_byte <= start_byte)
-	return NULL;
-    return vim_strnsave(str + start_byte, end_byte - start_byte);
-}
-
 /*
  * Handle:
  * - expr[expr], expr[expr:expr] subscript