Some Collection goodness.

assocations, keys, values + do in Dictionary et al.
numeric indexes using js methods moved from Collection
  to SequenceableCollection.
Different migrateClass:, uses atomic name-swap.

It fixes, partly, #382. It is possible to reparent
Set and HashedCollection. But reparenting SequenceableCollection
fails while moving Array.
Merge pull request #381 from herby/collection-goodness

Some Collection goodness.
Merge pull request #385 from herby/atomic-swap-migrate

Different migrateClass:, uses atomic name-swap.
Better protocols for ClassBuilder methods