Lazily find first element of filter()
Lazily find first element of filter()

I believe the right approach is to not bind any references to elements
until one actually starts iterating. This way the iterator can be passed
around before iteration starts without a dangling reference. This should
actually fix the filter() issues of #57 and #59

ryanhaining/cppitertools,ryanhaining/cppitertools,ryanhaining/cppitertools