go spec: adjust notion of Assignability

go spec: adjust notion of Assignability

This change makes it legal to pass a struct value as receiver
outside the package declaring the struct even if the respective
struct type has non-exported fields.

This is a backwards-compatible language change motivated by the
fact that it is already possible to circumvent the assignment
restriction when calling methods through interfaces (see issue
1402).

R=r, rsc, iant, ken2
CC=golang-dev
https://golang.org/cl/3926044