Tidy up samples to make them more kotlin-playground-friendly

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kotlinx-io was recently added to Kotlin Playground.

Some samples could not be executed there directly due to missing imports.
Others, when executed, prints nothing (as they only have asserts). Perhaps, we should follow a path taken by Kotlin Stdlib and replace asserts with print statements in rendered samples.

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Imports will be fixed here: #382

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Some samples use unsigned integer types to represent internal state (CRC32, for example), which contradicts guidelines on how these types should be used.