Only exception is RefChecks which produces errors when run
at end of group. Probably because it clashes with ExtensionMethods.

Only exception: elimByName, which produces errors if run at group end.

LambdaLift cannot run at same phase as Flatten, so it's better to move
Flatten one phase later.

Some check files that dependent on the order in which symbols were accessed
needed to be updated.

The other phase, LinkScala2Impls cannot yet be run at group end - it produces
errors if one tries.

Instead of restoreScopes, use elimStaticThis, which is the new
end of the group.

If all phases run at group end, any phase that adds or removes method parameters
makes all phases run before it in the same group have the wrong number of parameters
when type assigning an Apply or TypeApply. Previously this was allowed only if
relaxedTyping was true. Now it is allowed everywhere. We should still be able to
catch any errors in the transformer logic using Ycheck.

With this change, LinkScala2Impls can now be run at group end.

Move to end of group. Needed a fix in ExtensionMethods to work
correctly. Before the fix, extension methods got wrong flags.

We now have all miniphases running at group end. We can remove the infrastructure
to enable other behavior. If some parts of some transformations need to run at a different
phase than group end, we can always to `ctx.atPhase { ... }`.

It's no longer needed, since it's the default now.

Got deleted by accident before.

TreeTransform used cpyBetweenPhases, but TreeTransformer did not.
This does not seem sensical. Most likely this was overlooked. TreeTransform/TreeTransformer
is too similar, one gets confused easily what is what.