interp: add support of interface wrappers to type assertions

Store the interpreter value of the interface object to wrap as
a field called IValue, at offset 0 in wrapper structures.

Update extract to include IValue field.

In typeAssert, detect interface wrapper, and dereference the
interpreter value from IValue wrapper field.

Fixes #1166.

stdlib/go1_16_go_constant.go

@@ -61,6 +61,7 @@ func init() {
 
 // _go_constant_Value is an interface wrapper for Value type
 type _go_constant_Value struct {
+	IValue       interface{}
 	WExactString func() string
 	WKind        func() constant.Kind
 	WString      func() string