`NewTrust`, `NewPeerToPeerTrust` and `NewGlobalTrust`
Document field values of instance constructed via
`NewTrust`, `NewPeerToPeerTrust` and `NewGlobalTrust`.
Assert the values in corresponding unit test.
Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>
`PeerToPeerTrust` and `GlobalTrust` to nil message
Document that `Trust.ToV2`, `PeerToPeerTrust.ToV2`
and `GlobalTrust.ToV2` methods return `nil`when
called on `nil`. Document that `TrustFromV2`,
`PeerToPeerTrustFromV2`and `GlobalTrustFromV2`
functions return `nil` when called on `nil`. Write
corresponding unit tests.
Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>
