Improve ergonomics for qualifier keys

[matt-phylum]

Overview

I was writing some code to remove qualifiers from PURLs and found that you can't do purl.into_builder() and then remove all the qualifiers through builder methods without cloning the Purl or its qualifiers and then removing them one at a time, and even then the generics and type conversions for QualifierKey would get in the way of calling .without_qualifier or .with_qualifier using the keys of an existing Purl.

This PR

• allows removing a qualifier using just any AsRef<str>, even one that isn't Into<SmallString>, because removing a qualifier never creates a string.
• adds some impls for QualifierKey so that it the QualifierKey or &QualifierKey of an existing Purl can be used as a parameter to with_qualifier and without_qualifier.
• adds a without_qualifiers that removes all qualifiers.

It might be nice in some cases to expose Qualifiers::retain somehow via PurlBuilder but I haven't done it in this PR. It is already possible to do that (and everything else added in this PR) by accessing the qualifiers directly:

let mut builder = purl.into_builder();
builder.parts.qualifiers.retain(|k, v| k == "checksum");
builder.build()

Checklist

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• Have you created sufficient tests?

Issue

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