Feature/raw scale encoding

[valentunn]

• Support for raw scale serialization
• Support for encoding regular enums to DictEnum.Entry with null associated value
• Improve error messages for enum decoding

Changes

Raw scale serialization

• Useful when we want to operate low level scale structures for some of the fields / arguments
• RawScaleValue can be used for fields
• AsRawScaleValue wrapper can be used for top-level serialization (e.g. storage keys / whole values)

Regular enum encoding to DictEnum.Entry with null associated value

• IntroducedAsDictEnum annotation that can be applied to enum class to signal that it should be encoded as DictEnum.Entry