Update XCM MultiLocation

[shawntabrizi]

@gavofyork suggested we update the MultiLocation syntax to:

#[derive(Clone, Eq, PartialEq, Ord, PartialOrd, Encode, Decode, Debug)]
pub struct MultiLocation {
	parents: u8,
	junctions: Junctions,
}

#[derive(Clone, Eq, PartialEq, Ord, PartialOrd, Encode, Decode, Debug)]
pub enum Junctions {
	/// The interpreting consensus system.
	Null,
	/// A relative path comprising 1 junction.
	X1(Junction),
	/// A relative path comprising 2 junctions.
	X2(Junction, Junction),
	/// A relative path comprising 3 junctions.
	X3(Junction, Junction, Junction),
	/// A relative path comprising 4 junctions.
	X4(Junction, Junction, Junction, Junction),
	/// A relative path comprising 5 junctions.
	X5(Junction, Junction, Junction, Junction, Junction),
	/// A relative path comprising 6 junctions.
	X6(Junction, Junction, Junction, Junction, Junction, Junction),
	/// A relative path comprising 7 junctions.
	X7(Junction, Junction, Junction, Junction, Junction, Junction, Junction),
	/// A relative path comprising 8 junctions.
	X8(Junction, Junction, Junction, Junction, Junction, Junction, Junction, Junction),
}

And then we eliminate the Parent junction. This would force that all MultiLocations are always in their canonical/normalized form.