Rename Queue to Deque

'Queue' is not an accurate name any more; the correct name for the data structure
is a 'Deque'. We will add documentation to make it clear that a Deque works as either
a queue or a stack.
Correct documentation for Set (as regards behavior of #add for duplicate elements)
Clarify documentation for Set#sample
Make comment for Set#each_index clearer
Fix bug in Yardoc for SortedSet#include?