I get that it's the index of the last code point, but mainly I was surprised that
buf = IOBuffer()
write(buf, 0xf0, 0x9f, 0x9b, 0x80, 0xf0, 0x9f)
s = takebuf_string(buf)
convert(Vector{UInt8},s[chr2ind(s, length(s)):end])
returns [0xf0] not [0xf0, 0x9f]
