Add nullref

[rossberg]

Bring back the nullref type.

• In binary, it's a heaptype encoded as -0x12 (previously used for anyref, now unused).
• In text, nullref is shorthand for (ref null none), where none is the name for the bottom heap type (chosen as opposite to any).

No changes needed to the interpreter other than supporting it in binary and text format and allowing it in the validator. The rest of the change is merely renaming and reordering.

[titzer]

I think it'd be best to avoid reusing the code for anyref just yet., in particular we may want to revisit #271. That introduced a mode where an engine still has to model externref to pass the old spec and also model anyref to pass the GC spec.

[rossberg]

@titzer, if you need that mode flag anyway, wouldn't it trivially enable interpreting the code accordingly as well?

[titzer]

@rossberg I think it will be safer to use a new code to separate this PR (which is completely reasonable) from that other discussion.

[rossberg]

Okay, I changed the code to 0x1b for now, assuming that we'll eventually do a code space cleanup anyway. PTAL.

[titzer]

lgtm