Use unsigned integers when stable

[twyatt]

The RFC defines numeric types as unsigned; in KoAP (to prevent overflow) the next larger numeric type is used (e.g. a Long is used to store an unsigned int32).

unsigned integers would simplify a lot of internal code as well as simplify the public facing API. unsigned integers should be used when no longer experimental.

[DavidJRobertson]

Stable as of 1.5 https://kotlinlang.org/docs/whatsnew15.html#stable-unsigned-integer-types

[twyatt]

Stable as of 1.5 https://kotlinlang.org/docs/whatsnew15.html#stable-unsigned-integer-types

@DavidJRobertson thanks for the heads up!