Change also_known_as type to OrderedSet

[cycraig]

Description of change

Changes the CoreDocument::also_known_as field from Vec<Url> to OrderedSet<Url> to comply with the DID specification: https://www.w3.org/TR/did-core/#also-known-as

The alsoKnownAs property is OPTIONAL. If present, the value MUST be a set where each item in the set is a URI conforming to [RFC3986].

where "set" is defined to be an OrderedSet: https://infra.spec.whatwg.org/#ordered-set .

Besides the API changes, this may break DID Documents which have duplicate entries in their alsoKnownAs section.
This also adds IotaDocument::also_known_as_mut() since previously there was no safe way to update the field from IotaDocument without going through the unsafe core_document_mut.

Changed

• Change CoreDocument::also_known_as field from Vec<Url> to OrderedSet<Url>.
• Change CoreDocument::also_known_as() method return from &[Url] to &OrderedSet<Url>.
• Change CoreDocument::aslo_known_as_mut() method return from &mut Vec<Url> to &mut OrderedSet<Url>.
• Change IotaDocument::also_known_as() method return from &[Url] to &OrderedSet<Url>.

Added

• Add IotaDocument::also_known_as_mut() method.

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

[PhilippGackstatter]

Looks good to me!