InchCoord stand-in and init fixes

• Stand-in is now not a Class, but a reference to a variable (kClusterBaseClass) containing a reference to a Class.  This double-deref lets us cast and use it directly as an id.
• InchCoord's init cleaned up considerably to have two main execution paths: one if we receive the stand-in, and the other if we don't (e.g. if we have an alloc'd object to deal with).
	» init no longer touches `self` after the initial isClusterInstanceStandIn check (which only does an equality check).
	• The former execution path is now a InchCoord_initForStandIn class method, called directly on kClusterBaseClass.
	• The latter execution path is now a InchCoord_initForAllocedInstance instance method, called on self and acting prettymuch like a normal init.
	» Lots of comments about this wonkiness.

Also:
• Removed InchCoordPlaceholder.  Don't need that shit no mo ('s NSValue engineers are cray).  And it wasn't being used anywhere at all, since the stand-in change.
• Removed kClusterPlaceholderClass and isClusterPlaceholderClass as well.