Implement IUtf8SpanParsable on Guid

[SirCxyrtyx]

Part of #81500

API Proposal: #105653

This implements IUtf8SpanParsable on Guid by making the core of the parsing logic generic on TChar : IUtfChar<TChar>. A small amount of it is duplicated, with a version for char and byte, because doing so prevented a performance regression for char parsing.

In addition, it adds a Utf8 Parse and TryParse without the unused IFormatProvider parameter, to match how ISpanParsable was implemented. I did not add Utf8 overloads for ParseExact, but that can be discussed in the linked API proposal.

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[SirCxyrtyx]

@dotnet-policy-service agree

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[stephentoub]

Other than my comments/questions, LGTM. Thanks.

[SirCxyrtyx]

Just saw the updates, sorry for not responding quicker. Glad to see this merged!