Document field values of instance constructed via `NewChecksum`.
Assert the values in corresponding unit test.
Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>
Document that `Checksum.ToV2` method return `nil`
when called on `nil`. Document that `NewChecksumFromV2`
function return `nil` when called on `nil`. Write
corresponding unit tests.
Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>
