ByteString: provide ByteString-factory overload accepting unsigned bytes

[fzhinkin]

Currently, ByteString-factory accepts only signed bytes. Which is mildly annoying when it comes to writing code that initializes byte strings with some constants.

It would be nice to have an unsigned bytes overload.

// How it looks now
val magic = ByteString(0xcf.toByte(), 0xfa.toByte(), 0xed.toByte(), 0xfe.toByte())

// How should it look:
val magic = ByteString(0xcfu, 0xfau, 0xedu, 0xfeu)