Decrease memory usage of iterator implementations

[davidanthoff]

Quite a number of the Query iterators allocate and populate a result array in the start function call and essentially perform all the work of the iterator in the start function. Many of these could be rewritten to not use such a result array, and instead get elements from their source iterator in calls to the next function.

Such rewrites should be done for these iterators:

• EnumerableGroupJoin
• EnumerableGroupBy
• EnumerableSelectMany
• EnumerableJoin