Replace "collection" with "collections" in various file names since
we typically use the plural form in conversation and documentation.
Add set() and removeAt() methods needed for collection interfaces. Also
add the KeyedIterable and KeyedIterator interfaces.
Add __construct() methods for collections
This change is mostly for FB internal organizational reasons.
Building is not effected beyond the fact that the target now
lands in hphp/hhvm/hhvm rather than src/hhvm/hhvm.