don't prefetch next item in loop context

[davidism]

fixes #555

Previously, LoopContext prefetched the next item in the iterable, the internal iterator was advanced one further than the index. This caused issues with groupby or other iterators where items depend on the state of the iterator.

Now, the iterator will only advance early for the length, revindex, last and nextitem properties, using any other properties in the loop will not mess up groupby.

Unfortunately, this couldn't carry over to AsyncLoopContext directly. To look ahead in async, you need to await it.__anext__() rather than next(it). To await, you need to be in async def, but only __anext__ is async, not the properties, and we're not prefetching during iteration anymore. So this reimplements some properties to be async. This has the extra effect of making length, revindex, and revindex0 work for async iterables (they raised a TypeError before). The compiler must add await auto_await() around attribute access for these properties to resolve, previously it was only added for calls.

I feel like this implementation is cleaner overall, but I'm not sure of the implications for async performance.