interp: fix retrieving the string value of an interface

Thanks to @bailsman for providing first insight, in addition to
raising the issue.

Fixes #1342.

_test/issue-1342.go

@@ -0,0 +1,12 @@
+package main
+
+import "fmt"
+
+func main() {
+	var a interface{}
+	a = "a"
+	fmt.Println(a, a == "a")
+}
+
+// Output:
+// a true

interp/value.go

@@ -595,5 +595,10 @@ func genComplex(n *node) func(*frame) complex128 {
 
 func genValueString(n *node) func(*frame) (reflect.Value, string) {
 	value := genValue(n)
+
+	if n.typ.TypeOf().Kind() == reflect.Interface {
+		return func(f *frame) (reflect.Value, string) { v := value(f); return v, v.Elem().String() }
+	}
+
 	return func(f *frame) (reflect.Value, string) { v := value(f); return v, v.String() }
 }