Rename MethodQuery to DIDUrlQuery, move OrderedSet, KeyComparable

[cycraig]

Description of change

House-keeping changes: moves the OrderedSet collection and associated KeyComparable trait from identity-did to identity-core. This also makes OrderedSet more generic by introducing the Queryable trait for DID-specific lookups.

Breaking change due to different import paths and the introduction of Queryable to handle the query/query_mut functions. Also renames MethodQuery to a more generic DIDUrlQuery, since I intend to use it for a future resolve_service function.

Changed

• Move OrderedSet from identity-did to identity-core.
• Move KeyComparable from identity-did to identity-core.
• Rename MethodQuery to DIDUrlQuery and move its sub-package from /method to /utils.
• Rename KeyComparable::as_key() to KeyComparable::key().

Added

• Add Queryable trait.

Links to any relevant issues

Fixes #633.

Type of change

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

Unit tests and examples pass locally.

Change checklist

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[cycraig]

Not entirely happy with all changes, such as:

1. the naming of DIDUrlQuery (still needs to be something that can apply to querying a service or verification method but it really only uses the fragment part of the relative URL).
2. the naming of the identity-did/utils sub-module.
3. the fact that the key-uniqueness of OrderedSet can be violated by query_mut: this is possible in the original code even without these changes through IotaDocument/CoreDocument::resolve_method_mut which use it. This is why collections like HashSet do not allow mutable iterators or accessors, to avoid this problem. To fix it would mean removing any function that returns a mutable reference into an OrderedSet, which would not be so bad in my opinion (the intended usage is through methods like OrderedSet::replace for modification) but would be a significant breaking change.

[PhilippGackstatter]

Looks good to me!

I guess a more general name for DidUrlQuery would be DidDocumentPropertyQuery but that's unwieldy, so the current name seems perfectly fine to me.

Regarding the mutability: While annoying that it is possible, it's unlikely to happen, as someone would have to change the CoreDIDUrl of a e.g. method in the OrderedSet, iiuc. The account also doesn't expose that except through update_document_unchecked, so it's well contained.