Validate UTF-8 encoding/decoding correctness

[fzhinkin]

kotlinx-io uses a custom UTF-8 encoder and decoder implementations.

As part of library stabilization, we need to ensure that these implementations are fully UTF-8 conformant.

Unfortunately, UTF-8 does not provide any conformance test suite (only describes how one should look), so we have to figure out further details on our own.

An extra option here would be to support fuzzing-based tests.