[Grace-core] Type declarations

[James Noble]

> Good.  Hence my proposal that we write
> 
> 	const Boolean = type {&& (SelfType) -> SelfType, || (SelfType) -> SelfType, ... }

versus

	type Boolean = interface {&& (SelfType) -> SelfType; || (SelfType) -> SelfType; ... }

> const is like def, but you can't override it in sub-objects,  

I thought you didn't like "final"?

> and the value must be knowable at compile time.

right. which is certainly different to def...

J