this is not a user-visible change.

before, all interface values were

	struct Itype {
		Sigt *type;
		Sigi *inter;
		void *method[n];
	}

	struct Iface {
		void *addr;
		Itype *itype;
	}

the itype is basically a vtable, but it's unnecessary
if the static type is interface{ }.
for interface values with static type empty, the
new representation is

	struct Eface {
		void *addr;
		Sigt *type;
	}

this complicates the code somewhat, but
it reduces the number of Itypes that
have to be computed and cached,
it opens up opportunities to avoid function
calls in a few common cases,
and it will make it possible to lay out
interface{} values at compile time,
which i think i'll need for the new reflection.

R=ken
OCL=28701
CL=29121