compiler: fix crash with linked lists in interfaces

This commit fixes the following issue:
https://github.com/tinygo-org/tinygo/issues/309
Also, it prepares for some other reflect-related changes that should
make it easier to add support for named types (etc.) in the future.

testdata/interface.go

@@ -22,6 +22,10 @@ func main() {
 	println("Stringer.(*Thing).String():", itf.(Stringer).String())
 
 	println("nested switch:", nestedSwitch('v', 3))
+
+	// Try putting a linked list in an interface:
+	// https://github.com/tinygo-org/tinygo/issues/309
+	itf = linkedList{}
 }
 
 func printItf(val interface{}) {
@@ -134,3 +138,7 @@ func (p SmallPair) Print() {
 type Unmatched interface {
 	NeverImplementedMethod()
 }
+
+type linkedList struct {
+	addr *linkedList
+}