5843a6e3bc
Also tested (but not as fast):
```zig
pub fn utf8ByteSequenceLength(first_byte: u8) !u3 {
const len = @clz(u8, ~first_byte);
if (len == 0) return 1;
if (len < 4) return @intCast(u3, len);
return error.Utf8InvalidStartByte;
}
```