Update the Map_NestedLoops$Execute to work correct in one corner case
where the look ahead is disabled, but still returning the wrong result. 

Add a YamlPipelineSmallDT to mirrir the YamlPipelineOffDT, but with a
small lookahead setting. This was causing problems earier, and want to
keep the test to verify fixes. 

Create a separate yaml test for a small/no lookahead failure.
Refactor the data structure(s) used by GroupLookup_Default to be a
simple array of objects, with nested cursors/HKeys arrays. Previously
this was four separate arrays, clearly as a set of micro optimizations.
The new version tries to make it clearer what the manipluation structure
is, and is easier to follow in the debugger.
No, really, this time fix Map_NestedLoops$Execute to correctly work with
the failing test cases.