Add cycle detection when inserting quads from structs

[phyrwork]

Description

Cayley gets stuck in a recursive loop when trying to insert quads from a struct that (eventually) points to itself

Steps to reproduce the issue:

e.g. (based on one of the examples)

type Person struct {
	// dummy field to enforce all object to have a <id> <rdf:type> <ex:Person> relation
	// means nothing for Go itself
	rdfType   struct{} `quad:"@type > ex:Person"`
	//ID        quad.IRI `json:"@id"`     // tag @id is a special one - graph node value will be stored in this field
	Name      string   `json:"ex:name"` // field name (predicate) may be written as json field name
	Age       int      `quad:"ex:age"`  // or in a quad tag
	Relatives []*Person `quad:"ex:relative"`
}

bob := Person{
	//ID:   quad.IRI("ex:bob").Full().Short(),
	Name: "Bob", Age: 32,
}
jane := Person{
	//ID:   quad.IRI("ex:bob").Full().Short(),
	Name: "Jane", Age: 32,
}

bob.Relatives = []*Person{
	&jane,
}
jane.Relatives = []*Person{
	&bob,
}

fmt.Printf("saving: %+v\n", bob)
id, err := config.WriteAsQuads(writer, bob)
checkErr(err)
fmt.Println("id for object:", id) // should be equal

Received results:

=== RUN   Test_SchemaExample
saving: {rdfType:{} Name:Bob Age:32 Relatives:[0xc420078720]}
runtime: goroutine stack exceeds 1000000000-byte limit
fatal error: stack overflow

Expected results:

Nodes 'bob' and 'jane' added to graph without error

Output of cayley version or commit hash:

945c4253f890b0048a31da7c4a8a7c06625457c7

Environment details:

Backend database: MemoryGraph