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.

src/runtime/func.go

@@ -16,13 +16,13 @@ type funcValue struct {
 
 // funcValueWithSignature is used before the func lowering pass.
 type funcValueWithSignature struct {
-	funcPtr   uintptr // ptrtoint of the actual function pointer
-	signature *uint8  // pointer to identify this signature (the value is undef)
+	funcPtr   uintptr     // ptrtoint of the actual function pointer
+	signature *typecodeID // pointer to identify this signature (the value is undef)
 }
 
 // getFuncPtr is a dummy function that may be used if the func lowering pass is
 // not used. It is generally too slow but may be a useful fallback to debug the
 // func lowering pass.
-func getFuncPtr(val funcValue, signature *uint8) uintptr {
+func getFuncPtr(val funcValue, signature *typecodeID) uintptr {
 	return (*funcValueWithSignature)(unsafe.Pointer(val.id)).funcPtr
 }

src/runtime/interface.go

@@ -43,7 +43,16 @@ type interfaceMethodInfo struct {
 	funcptr   uintptr // bitcast from the actual function pointer
 }
 
-type typecodeID struct{}
+type typecodeID struct {
+	// Depending on the type kind of this typecodeID, this pointer is something
+	// different:
+	// * basic types: null
+	// * named type: the underlying type
+	// * interface: null
+	// * chan/pointer/slice: the element type
+	// * array/func/map/struct: TODO
+	references *typecodeID
+}
 
 // Pseudo type used before interface lowering. By using a struct instead of a
 // function call, this is simpler to reason about during init interpretation