simplify expr by merging all simple eval functions
into a single eval interface{}.
R=austin
DELTA=1597 (730 added, 780 deleted, 87 changed)
OCL=34182
CL=34198
compilation to put the definitions in reasonable places. No
code changes, just movement.
R=rsc
APPROVED=rsc
DELTA=479 (232 added, 247 deleted, 0 changed)
OCL=34067
CL=34069