gob: clean up getTypeInfo (address a TODO) also fix a caching bug.
gob: clean up getTypeInfo (address a TODO)
also fix a caching bug.

R=rsc
CC=golang-dev
http://codereview.appspot.com/4261049

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang