Fix encoding of slices.
Fix encoding of slices.

In 8fe0726f, the meaning of a slice's base and root have changed. They
are now offsets from the pool's base. Fix the serialization code to no
longer attempt to adjust the slice's root and base, since they no longer
are absolute addresses.

Fixes #2051

google/agi,google/agi,Qining/gapid,google/gapid,google/agi,google/gapid,google/agi,google/gapid,Qining/gapid,google/agi,google/gapid,google/agi,Qining/gapid,pmuetschard/gapid,Qining/gapid,google/gapid,google/agi,pmuetschard/gapid,google/gapid,google/gapid,pmuetschard/gapid,Qining/gapid,Qining/gapid,google/gapid,Qining/gapid,pmuetschard/gapid,pmuetschard/gapid,pmuetschard/gapid,google/agi,pmuetschard/gapid,pmuetschard/gapid,Qining/gapid