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://codereview.appspot.com/7200046
go/types: expose types.IsIdentical, the Type equivalence relation.

This function is absolutely critical for clients such as
exp/ssa, and too complex for clients to duplicate.

As with CL 7200046, gri expressed in the doc below [gophers
only] before going on leave that he intended to expose such a
predicate, though his wording suggests as an interface method
of Type rather than a standalone function.  (My preference is
for binary methods to be standalone; see "On Binary Methods",
Kim Bruce, 1995).  In any case if he wishes to move it that's
easily accommodated by clients.

https://docs.google.com/a/google.com/document/d/1-DQ4fxlMDs9cYtnkKhAAehX6MArjOQyJsRXp-6kiJLA/edit#heading=h.k3bwja7xony9

R=iant, gri, iant
CC=golang-dev
https://codereview.appspot.com/7203051