Allow Code Metadata Annotations to have arbitrary hex strings

[jabraham17]

This PR adds support for arbitrary hex strings in code metadata annotations. Previously, the decoded strings would be checked as utf-8 strings. However, this prohibits something like (@metadata.code.trace_inst "\FF\FF") since 0xffff is not a valid utf-8 string.

Also modified existing roundtrip test to have a string that includes a non-utf8 character

[keithw]

Thank you -- merging as this was approved and also lgtm.