Add eager and lazy statement prefixes.

For now, lazy does nothing, but it will mean we can write "lazy for
@x { ... }".
Don't rely on unreliable return value.
Make statement-level for @x { ... } always sink.
Bring statement-modifying for in line with spec.

It now sinks at statement level also.
Fix combinations implementation's use of for.
Remove sink forcing; rely on improved code-gen.
Get NQP improvements for loop-cleanup.