go/types: modify Eval, EvalNode to return a TypeAndValue

The new interface makes the functions more useful by allowing clients to
check the various properties that TypeAndValue provides.

Change-Id: I8b41a27316081bea24a18ffe6fa1812e809d6f67
Reviewed-on: https://go-review.googlesource.com/2134
Reviewed-by: Robert Griesemer <gri@golang.org>