Define and implement behavior for `@Unroll`ing abstract methods

[lihaoyi]

Should we generate additional abstract methods, and assume the user will @Unroll the corresponding implementation? Or should we generate additional concrete forwarders so the implementation does not need to be unrolled?

[lihaoyi]

The current behavior is that we generate concrete forwarders even if the @Unrolled method is abstract.

This can cause weird errors if you also @Unroll the concrete implementation, because the two sets of concrete forwarders conflict

We should check and handle this case

[lihaoyi]

done