Change document controller type to OneOrSet

[cycraig]

Description of change

Allows multiple controllers to be set on a CoreDocument using a new generic OneOrSet collection, which wraps OrderedSet. OneOrSet is similar to OneOrMany except all entries are unique by key and is guaranteed to be non-empty.

This is to comply with the W3C DID specification:

The controller property is OPTIONAL. If present, the value MUST be a string or a set of strings that conform to the rules in § 3.1 DID Syntax.

This type change is backwards compatible since a single controller DID will deserialize to OneOrSetInner::One.

Changed

• Change CoreDocument::controller() from Option<&CoreDID> to Option<&OneOrSet<CoreDID>.
• Change CoreDocument::controller_mut() from Option<&mut CoreDID> to &mut Option<OneOrSet<CoreDID>>.

Added

• Add OneOrSet collection.
• Add IotaDocument::set_controller().

Links to any relevant issues

See issue #306.

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

Added new unit tests to ensure adding and removing document controllers is supported and works with diffs.

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, thanks for the changes!

[olivereanderson]

This is a nice improvement to the code base, but I think the associated from<U>(other: OneOrSet<U>) method should be refactored before this gets merged.

[olivereanderson]

Thanks for the changes.