- simplified assignment checking by removing duplicate code
- implemented field lookup (methods, structs, embedded fields)
- importing methods (not just parsing them)
- type-checking functions and methods
- typechecking more statements (inc/dec, select, return)
- tracing support for easier debugging
- handling nil more correctly (comparisons)
- initial support for [...]T{} arrays
- initial support for method expressions
- lots of bug fixes

All packages under pkg/go as well as pkg/exp/types typecheck
now with pkg/exp/gotype applied to them; i.e., a significant
amount of typechecking works now (several statements are not
implemented yet, but handling statements is almost trivial in
comparison with typechecking expressions).

R=rsc
CC=golang-dev
https://golang.org/cl/6768063