Add load_instance_mode

[Simn]

This passes an additional argument to all type instance loading operations, which currently has three possible values:

type load_instance_mode =
	| LoadNormal
	| LoadReturn
	| LoadAny (* We don't necessarily know why we're loading, so let's just load anything *)

I primarily want to use it to disallow loading Void in places that aren't return types, but there might be some other applications for this as well. Shouldn't have any observable impact.