std: optimise utf8ByteSequenceLength
std: optimise utf8ByteSequenceLength

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;
}
```

zig-lang/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,andrewrk/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig,raulgrell/zig