Fix various compiler warnings.

Fix various warnings produced by Clang:

Array.h:32:25: Declaration shadows a field of 'ArrayData<TYPE>'
SegmentInfoCollection.h:32:10: 'Lucene::SegmentInfoCollection::equals' hides overloaded virtual function
Searchable.h:24:14: 'Lucene::Searchable' has virtual functions but non-virtual destructor
Fieldable.h:20:14: 'Lucene::Fieldable' has virtual functions but non-virtual destructor

(a8bac06c4c4997310633c8ff13952d28b1c67593 helped a lot with warnings, these remaining few are shown when including Lucene headers from other code)