go spec: restrictions for index and slice expressions
go spec: restrictions for index and slice expressions

At the moment, gc and gccgo report compile-
time errors for certain constant indexes that
are out of bounds. The spec however requests
a run-time panic for out-of-bounds indexes
(http://tip.golang.org/ref/spec#Indexes).

Document the status quo.

Fixes issue 4231.

R=r, rsc, iant, ken
CC=golang-dev
http://codereview.appspot.com/6699048

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