Generate DID URL from components

[tomdaffurn]

• Remove URL field from DID type. It was inconsistently populated, and could differ from URI
• Add a function to calculate the URL from the URI, params, path, query and fragment fields

The result is that URL() will always be consistent with URI, while allowing DID URLs to be parsed and retain all of their information. Value/Scan and String/Parse have symmetrical behaviour so all DIDs can be marshalled and unmarshalled reliably.

Closes #164 #163