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
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go