Added constants to Vector for the primary axes

Made constants for the X, Y, and Z axes, all assuming 3-space. Need to come up with something more generic, but that's a commit for another day.
Fixed bad copypasta in Obround
Removed support for using square brackets to create a Rectangle

Switched to using named arguments
Fixed PointZero equality

It's a dirty hack, but it works (for now)
SizeRectangle#origin defaults to PointZero