node/bindnode: start of a reflect-based Node implementation

Lots of TODOs and polishing to do, but it passes the schema/gen/go tests
with minimal changes.

Follow-up commits will continue filling in the gaps and adding
documentation, notably examples. We'll also make the tests run by
default, after a bit of refactoring.

node/bindnode/example_test.go

+package bindnode_test
+
+import (
+	"os"
+
+	ipld "github.com/ipld/go-ipld-prime"
+	"github.com/ipld/go-ipld-prime/codec/dagjson"
+	"github.com/ipld/go-ipld-prime/fluent/qp"
+	"github.com/ipld/go-ipld-prime/node/bindnode"
+	"github.com/ipld/go-ipld-prime/schema"
+	"github.com/polydawn/refmt/json"
+)
+
+func ExamplePrototypeOnlySchema() {
+	ts := schema.TypeSystem{}
+	ts.Init()
+	ts.Accumulate(schema.SpawnString("String"))
+	ts.Accumulate(schema.SpawnInt("Int"))
+	ts.Accumulate(schema.SpawnStruct("Person",
+		[]schema.StructField{
+			schema.SpawnStructField("Name", "String", false, false),
+			schema.SpawnStructField("Age", "Int", true, false),
+			schema.SpawnStructField("Friends", "List_String", false, false),
+		},
+		schema.SpawnStructRepresentationMap(nil),
+	))
+	ts.Accumulate(schema.SpawnList("List_String", "String", false))
+
+	schemaType := ts.TypeByName("Person")
+	proto := bindnode.PrototypeOnlySchema(schemaType)
+
+	n, err := qp.BuildMap(proto, -1, func(ma ipld.MapAssembler) {
+		qp.MapEntry(ma, "Name", qp.String("Michael"))
+		qp.MapEntry(ma, "Friends", qp.List(-1, func(la ipld.ListAssembler) {
+			qp.ListEntry(la, qp.String("Sarah"))
+			qp.ListEntry(la, qp.String("Alex"))
+		}))
+	})
+	if err != nil {
+		panic(err)
+	}
+	nr := n.(schema.TypedNode).Representation()
+	dagjson.Marshal(nr, json.NewEncoder(os.Stdout, json.EncodeOptions{}), true)
+
+	// Output:
+	// {"Name":"Michael","Friends":["Sarah","Alex"]}
+}