A callback loop version emits its loop twice. A loop inside either copy
could be versioned again, so a nest of loops that each call a callback
built just before them (`const g = adder(i)`) doubled at every level: a
three-deep nest was emitted seven times per function.
A loop inside a callback version, or the versioned loop itself emitted
again for each copy, is no longer versioned, so a loop nest is emitted at
most twice. The outer loop keeps its version; inner loops keep their
calls as they are.
Chris Tate
cd2b1e2574