wip(amino): demo bug with TypeInfo.String

Add a test that demonstrates a bug when the underlying type represented
by TypeInfo has fields of the same type: TypeInfo.String() triggers a
stack overflow because the codec reuses the same TypeInfo instance when
affecting the master TypeInfo.Fields.

This happens when cdc.fullnameToTypeInfo is printed (in case of error
for instance see codec.go:535), causing all registered TypeInfo to be
printed too, which calls their String method.

Example of type that can triggers this issue:
tm2/pkg/crypto/merkle.SimpleProofNode.

tm2/pkg/amino/typeinfo_test.go

@@ -0,0 +1,15 @@
+package amino
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestTypeInfoString(t *testing.T) {
+	type T struct {
+		N string
+		T *T
+	}
+	typeInfo := gcdc.newTypeInfoUnregisteredWLocked(reflect.TypeOf(T{}))
+	_ = typeInfo.String()
+}