Toward Meadowcap-y PIO

[AljoschaMeyer]

Things we still need:

• a struct McReadCapability (a thin wrapper around McCapability which ensures an access mode of read)
• an impl ReadCapability for McReadCapability
• an impl EnumCapability for McEnumerationCapability
• a struct DecodedMcReadCapability consisting of a granted Area and a Namespace, with a signature-verifying RelativeDecodable implementation suitable for decoding it as part of a PioBindReadcapability message. Analogous to Add DecodedMcEnumCap with RelativeDecodable impl #126
• an impl ReadCapability for DecodedMcReadCapability