gob: updates to the debugging tool.

- complex numbers now supported.
- entirely independent of standard decode code.
- parser has no read-ahead; that is, the scanning works
simply by reading the values as they arrive, not by trying
to count bytes for message boundaries, a proof of concept
for the pending rewrite of the regular decoder.

R=rsc, r2
CC=golang-dev
https://golang.org/cl/4084044