go/types: add String() method to Type interface.

go/types: add String() method to Type interface.

All implementations delegate to typeString.

Though I don't wish to exploit gri's absence to change
his code, this change is pretty low-risk and he assented to it
in the blue ink in the doc below [gophers only].
https://docs.google.com/a/google.com/document/d/1-DQ4fxlMDs9cYtnkKhAAehX6MArjOQyJsRXp-6kiJLA/edit#

R=iant, gri, gri
CC=golang-dev
https://golang.org/cl/7200046