Add a SchemaConcatenate operation.

This stops one step shy of introducing an "include" features to th
IPLD Schema syntax... but can easily be used to produce such behavior.

schema/dmt/operations.go

@@ -0,0 +1,27 @@
+package schemadmt
+
+import (
+	"github.com/ipld/go-ipld-prime/datamodel"
+	"github.com/ipld/go-ipld-prime/node/bindnode"
+)
+
+// ConcatenateSchemas returns a new schema DMT object containing the
+// type declarations from both.
+//
+// As is usual for DMT form data, there is no check about the validity
+// of the result yet; you'll need to apply `Compile` on the produced value
+// to produce a usable compiled typesystem or to become certain that
+// all references in the DMT are satisfied, etc.
+func ConcatenateSchemas(a, b *Schema) *Schema {
+	// The joy of having an intermediate form that's just regular data model:
+	// we can implement this by simply using data model "copy" operations,
+	// and the result is correct.
+	nb := Type.Schema.NewBuilder()
+	if err := datamodel.Copy(bindnode.Wrap(a, Type.Schema.Type()), nb); err != nil {
+		panic(err)
+	}
+	if err := datamodel.Copy(bindnode.Wrap(b, Type.Schema.Type()), nb); err != nil {
+		panic(err)
+	}
+	return bindnode.Unwrap(nb.Build()).(*Schema)
+}