Revert r24960, "Change Types#narrow to create a...

Revert r24960, "Change Types#narrow to create an existential rather than
a refinement type."

For many days I have been haunted by the knowledge that the 20 seconds
I took off quick.comp between r24893 and r24920 all came roaring back
around r24960 for no reason which I could discern. To verify r24960 was
not at fault I had compared it against r24959 more than once, and the
difference was negligible.

It was negligible, that is, until I compared under -optimise, and then
the wheels came off. In fact the differences there are so spectacular
(quick.lib under -optimise goes from 8:50 to 4:26 with this patch,
quick.comp from 8:34 to 6:30) that there must be some major disconnect
between what the benchmark charts are measuring and what I am measuring.

In any case, the charts and I do agree on the direction of the arrow.
For reasons which remain to be determined, this commit was bad on the
nanos. Review by odersky.

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