Support mutually recursive types

[gotwarlost]

Self-recursion works.

$ rdl version
rdl 1.4.7 
$ cat x.rdl
type Node Struct {
    Node left;
    Node right;
}
$ rdl -ps generate json x.rdl 
{
    "name": "x",
    "types": [
        {
            "StructTypeDef": {
                "type": "Struct",
                "name": "Node",
                "fields": [
                    {
                        "name": "left",
                        "type": "Node"
                    },
                    {
                        "name": "right",
                        "type": "Node"
                    }
                ]
            }
        }
    ]
}

However, mutual recursion doesn't

$ cat x.rdl
type Node Struct {
    Node left;
    Node right;
    Tree tree;
}

type Tree Struct {
    Node root;
}
$ rdl -ps generate json x.rdl 
*** Error (x.rdl, line 4): No such type: Tree
  1 type Node Struct {
  2     Node left;
  3     Node right;
  4     Tree tree;
  5 }
  6 
  7 type Tree Struct {
  8     Node root;
  9 }
 10 

[boynton]

Also, recursive references should only be valid if the fields are optional.
[see #23]

[boynton]

The mutual recursion described above is not usefully representable in JSON, i.e. the "tree" field of Node would be a complete copy of tree for every node. If you want to represent a network of references, you must define the references (i.e. UUIDs, URLs, or Strings), and use them to refer to common objects. RDL does not represent "references" to data, just structure.