go.tools/ssa/interp: improve print() on interfaces, and log message for panic.

R=gri
CC=golang-dev
https://golang.org/cl/21960043

ssa/interp/interp.go

@@ -547,7 +547,7 @@ func runFrame(fr *frame) {
 		fr.panicking = true
 		fr.panic = recover()
 		if fr.i.mode&EnableTracing != 0 {
-			fmt.Fprintf(os.Stderr, "Panicking: %s.\n", fr.panic)
+			fmt.Fprintf(os.Stderr, "Panicking: %T %v.\n", fr.panic, fr.panic)
 		}
 		fr.runDefers()
 		fr.block = fr.fn.Recover // recovered panic

ssa/interp/ops.go

@@ -23,6 +23,10 @@ type targetPanic struct {
 	v value
 }
 
+func (p targetPanic) String() string {
+	return toString(p.v)
+}
+
 // If the target program calls exit, the interpreter panics with this type.
 type exitPanic int
 

ssa/interp/value.go

@@ -395,7 +395,9 @@ func toWriter(w io.Writer, v value) {
 		}
 
 	case iface:
+		fmt.Fprintf(w, "(%s, ", v.t)
 		toWriter(w, v.v)
+		io.WriteString(w, ")")
 
 	case structure:
 		io.WriteString(w, "{")