* 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