ADR14: Metadata changes

[matevz]

Changes to ADR 14:

• Combine Metadata and Message fields into a single CBOR-encoded object with metadata and tx fields respectively.
• Fix metadata.chain_context encoding.

Rationale:

1. You need to prase CBOR message in order to determine its size.
2. CBOR wrapped inside another CBOR is CBOR. This means you don't need any additional memory (as is the case, for example, in Base64) in order to parse the CBOR-encoded object (e.g. transaction) wrapped inside CBOR.

As a consequence, there is no added value, if we pass two separate Meta+Message CBOR-encoded messages to the hardware wallet, so we combine them inside a single Data CBOR-encoded map with metadata and tx fields.

[matevz]

The new version of nano app supports detection of metadata size, so there is no need to combine metadata+tx into a single object anymore. Closing.