json2: reorganize encode string

[enghitalo]

Next PRs

• fix buf.grow_cap(s.len) or make buf.ensure_cap(buf.len + s.len) public
• find a way to improve the code bellow

if char_len == 1 {
	buf << chr // this is very slow, of course
} else {
	handle_multi_byte_char(e, mut buf, s[i..i + char_len])
}