Operator flat map rewrite

[akarnokd]

An alternative to #2928 by using flatMap(o -> o) == merge() identity.

Benchmark:

It is slightly better than #2928 for flatMap() benchmarks and slightly worse than merge()

[benjchristensen]

What do the columns represent in that benchmark?

[akarnokd]

1.x = current head,
1st try = rewrite but only with the scalar optimizations,
3rd try = rewrite+scalar optimizations + unbounded mode optimizations,
flatMap = merge implemented with flatMap instead the other way around so merge(map(func)) inlines func into merge and saves on a lift() call.

[davidmoten]

I made this comment on your tool's site but I think we can't assess whether the benchmarks indicate significant change unless the error value is taken into account and I assume these visualizations always ignore the error value (is that right?).

[akarnokd]

I'm closing this in favor of the recent changes in #2928.