This is a manual undo of CL 5674098.
It does not implement the even less strict spec
that we just agreed on, but it gets us back where
we were at the last weekly.
R=ken2
CC=golang-dev
https://golang.org/cl/5683069
* Code for assignment, conversions now mirrors spec.
* Changed some snprint -> smprint.
* Renamed runtime functions to separate
interface conversions from type assertions:
convT2I, assertI2T, etc.
* Correct checking of \U sequences.
Fixes#840.
Fixes#830.
Fixes#778.
R=ken2
CC=golang-dev
https://golang.org/cl/1303042