proper elementClass for WrappedArray

(backport of 45ef0514e, part 1)
avoid reflect overhead of certain array instantiations

the manifests for Any, Object/AnyRef, AnyVal, Null and Nothing
now have their `newArray` methods overridden to avoid reflective overhead
of array instantiation.

(backport of 45ef0514e, part 2)
Merge pull request #1842 from adriaanm/backport-1821

Backport 1821