Interop between AS types and common CLR types

[warriordog]

Various tweaks and small refactors to improve compatibility between AS and .NET. Depends on changes in #8.

Create new types:

• ASUri (use ASUriConverter)
• LinkRel (use LinkRelConverter)

Implement these implicit casts between AS / CLR types:

• ASUri <-> string, Uri
• ASLink <-> ASUri, Uri
• MentionLink <-> ASUri, Uri, string
• Linkable<T> <-- ASLink, ASUri, Uri, string, T
• Linkable<T> --> ASLink?, T?
• LinkRel <-> string
• ASCollection <-- List<T>, T
• ASOrderedCollection <-- List<T>, T
• ASCollectionPage <-- List<T>, T
• ASOrderedCollectionPage <-- List<T>, T

Implement these custom JsonConverters:

• ASUriConverter <-> JsonTokenType.String
• LinkRelConverter <-> string

Change the type of these properties:

• ASLink.HRef to type ASUri
• ASLink.Rel to type LinkRel

REMOVE these implicit casts:

• NaturalLanguageString <-> string (this will be redesigned in Improve design of NaturalLanguageString #12)

Additionally, add unit tests for all new types and behavior.

[warriordog]

Duration is going to be a brutal pain to implement, so dropping it from this issue.

[warriordog]

Done! 🎉