BREAKING CHANGE: StellarAssetType is renamed to StellarAsset.
The name StellarAssetType is reused for the corresponding enum.
Enums are introduced in several other places. Their values correspond to
allowed values of (previously int) fields so this should not pose a
compatibility problem.
Many fields are now required. We believe that this should not pose a
compatibility problem, because all known interfaces to Stellar signing
actually accept Stellar XDR on input, whose required fields match the
protobuf schema.
