Introduce specific `Name` type

[ryangjchandler]

Right now, we use SimpleIdentifier for basically everything that is "named", e.g:

• Named arguments.
• Structure names.
• Function names.
• Labels
• Declare statements

This was a good way of getting things to work, but now that I'm trying to do more complicated things I'd like to introduce a new Name type. The difference between a Name and SimpleIdentifier is where they're used.

Name would be used for things like functions, classes, etc. The current IdentifierQualification stuff would be moved away from SimpleIdentifier and turned into NameQualification.

[ryangjchandler]

This has been completed through various commits and PRs.

Essentially, anywhere a class or type name can appear is now an instance of Name instead of SimpleIdentifier. The parser does a little bit of extra work too, resolving names where possible.

Example of name resolution:

use Foo\Bar\Baz;

$baz = new Baz();

The new Baz() part will have a target field containing:

Name {
	kind: NameKind::Resolved(ResolvedName {
		resolved: "Foo\Bar\Baz",
		original: "Baz"
	}),
	span: ...
}

Since the parser tracks use statements, it can correctly resolve the Baz part to the imported Foo\Bar\Baz class.