Resolve recursive type issue

[i-norden]

The current IPLD code gen creates recursive types (TrieNode) that won't compile.

This is also an issue for go-codec-dagcosmos (MerkleTreeNode,IAVLNode, and SMTNode are all recursive types).

[i-norden]

This was fixed for the types here manually to get things working and the tests passing, but we need to upstream a fix to the fundamental issue in go-ipld-prime. An issue to track that is opened at #20, it includes other code gen issues other than this recursive one.

[warpfork]

This is currently a poorly documented / alpha API issue, sorry about the discoverability --

If you've got a union in there somewhere, you can probably fix this with a bit of config which tells the codegen to use interfaces (which use pointers, and thus break the cycle problem), like this:

https://github.com/ipld/go-ipld-prime/blob/b7347f196aa5653fddb5631124f8821ce425bfc9/schema/gen/go/testUnionsKinded_test.go#L25

Alternatively, if you haven't got a union, but you've got a maybe (either a nullable or an optional, or both at once) somewhere, then you can use this kind of config (except set it to true, to get a pointer):

https://github.com/ipld/go-ipld-prime/blob/b7347f196aa5653fddb5631124f8821ce425bfc9/schema/gen/go/testStructsContainingMaybe_test.go#L25

If you don't have either of those -- well, you must, actually :) otherwise the schema would be describing a structure that would legitimaty be infinite in size, which would be a bad schema.

(We intended to put a better configuration interface on this thing, but are in a bit of a tumble with prioritization lately, unfortunately.)