Further performance improvements in sorting: reduce number of comparisons

Further performance improvements in sorting: reduce number of comparisons
during initial run formation by keeping both current run and next-run
tuples in the same heap (yup, Knuth is smarter than I am).  And, during
merge passes, make use of available sort memory to load multiple tuples
from any one input 'tape' at a time, thereby improving locality of
access to the temp file.