Explicitly specify handling of invalid UTF-8 in CBOR text strings and Cadence strings

[fxamacker]

The specification should be explicit about invalid UTF-8 strings.

My preference is to reject invalid UTF-8 text strings. However, there may be edge cases I'm not aware of, so confirm with Cadence team first before adding text.

[fxamacker]

Closed by PR #57 which specifies requirements for Valid and Deterministic Encodings.