Add extensions to integrate ByteString with Kotlin stdlib APIs

[fzhinkin]

Some Kotlin standard library APIs look natural to be integrated with ByteStings:

• Base64 (https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.io.encoding/-base64/)
• HexFormat (https://github.com/JetBrains/kotlin/blob/master/libraries/stdlib/src/kotlin/text/HexFormat.kt)

The kx-io library should provide extensions facilitating use of ByteString with these APIs.