Replace a call to mapAccumR, which uses linear stack space, with a gadget that
uses constant space.
Remove an unused parameter from fromOnDiskName.
The tests T1292_imports and T4239 are now reporting imported names in a
different order. I don't completely understand why, but I presume it is because
the symbol tables are now read more strictly. The new order seems better in
T1792_imports, and equally random in T4239.
There are several performance test improvements.