spec: allow comparison of structs, arrays containing comparable values
spec: allow comparison of structs, arrays containing comparable values

Also, clarify when interface comparison panics and
that comparison to nil is a special syntax rather than
a general comparison rule.

R=r, gri, r, iant, cw, bradfitz
CC=golang-dev
http://codereview.appspot.com/5440117

apparentlymart/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,gnanderson/go,dsriram/golang,tv42/old-go,gnanderson/go,apparentlymart/golang,tv42/old-go,tv42/old-go,dsriram/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,dsriram/golang