Changes to Url to get it more in line with URI spec

[tmenier]

Flurl.Url parses a URL string into its parts which it exposes as properties, namely Path, Query, and Fragment. These are roughly equivalent to the formally defined parts of a URI, with some major exceptions:

• Stuff on the front part of the URI (sheme, host, etc) are notably missing.
• BREAKING: Flurl's notion of Path is not at all consistent with the URI definition, as it includes everything except the query and fragment.

I'd like to get Flurl.Url a little closer to System.Uri, although that has at least one known quirk of its own: Authority does not include user info, if provided.

This change would (re)define the following properties on Url:

• Scheme (new)
• UserInfo (new)
• Host (new)
• Port (new)
• Path (breaking change)
• PathSegments (new)
• Query (same as current)
• Fragment (same as current)
• Authority (new, derived: UserInfo + HostName + Port)
• Root (new, derived: Scheme + Authority)

Also:

• new static Url.Parse method, which is just syntactic sugar for new Url(string)
• BREAKING: GetRoot() is removed, use Root property going forward

References (to make sure I get all the details right):
https://en.wikipedia.org/wiki/Uniform_Resource_Identifier#Definition
https://stackoverflow.com/questions/2142910/whats-the-difference-between-uri-host-and-uri-authority

[tmenier]

As a more complete alternative to System.Uri, I want to point out where Flurl.Url differs. In most cases these differences address what I believe are quirks, i.e. areas where System.Uri differs from RFC 3986:

• Uri.Query includes the ? character; Url.Query does not.
• Uri.Fragment includes the # character; Url.Fragment does not.
• Uri.AbsolutePath always includes a leading / character; Url.Path includes it only if it's actually present in the original string (e.g. for "http://foo.com", Url.Path is an empty string.)
• Uri.Authority does not include user info (i.e. user:pass@); Url.Authority does.
• Uri defaults the Port if not explicitly set; Url.Port is nullable and is not defaulted. (This is opinionated behavior, not necessarily a quirk fix.)
• Uri will make no attempt to parse a relative URL; Url assumes that if the string doesn't start with {scheme}://, then it starts with a path and parses it accordingly.