Make functions to read and write code points public

[fzhinkin]

Currently, there's no way to read a code point from a source containing UTF-8 encoded data or write a single code point to sink encoding it to UTF-8 along the way.

Corresponding functions exist but are hidden: #136 (comment)

It's time to make them public under the following names:

• Sink.writeCodePointValue(value: Int)
• Source.readCodePointValue(): Int

There is no Utf8 in the names for consistency with readString/writeString, but there is a Value suffix.
At some point, CodePoint type will be added to Kotlin Stdlib, so it'll be nice to preserve readCointPoint/writeCodePoint names for it. However, such a type most likely will have a value: Int property and the names mentioned above are referring to that property.