[Dbix-class] Collapse result rewrite merged to -current

[Matt S Trout]

This should mean the end of the annoying 'results disappearing' prefetch
bugs (it doesn't do anything about the LEFT -> INNER join issue for long
prefetches yet, that's a separate problem).

However, that should mean some of you will find prefetch usable in situations
you've been forced not to use it. Plus it no longer uses recursive sub calls,
it's all unrolled, so I'm hoping it might be a bit faster.

In theory.

In order to make this happen, I ended up rewriting the entirety of
_collapse_result, and while all tests now pass we almost certainly still
don't have enough of them, so can anybody feeling insane please install
-current, beat it with a shovel and tell us if anything's broken?

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