step 2 of the great buffer shift.

step 2 of the great buffer shift.
make strings.Buffer handle strings and bytes with comparable efficiency.
if ok, next step will be to move this code to bytes.Buffer and terminate
strings.Buffer's short happy life.

R=rsc
DELTA=292  (212 added, 0 deleted, 80 changed)
OCL=34837
CL=34849