To get this to work, a few changes had to be made:
1. The semantics of `ref` for JTDDataType had to be changed. I don't
   entirely understand why, but my guess is that infer steps allow the
   compiler to "take a break" and so this helps with the recursion
   checking.
2. Added `SomeJTDSchemaType`. This is necessary to prevent typescript
   from inferring a JTDDataType when actually it's a different schema.
   This is especially a problem for simple types, e.g. the empty schema
   is valid JTD so it can confuse things.

Three other notes about the current implementation. Historically
specifying a type for compile indicated that that was the return type.
To keep that working, the overload signatures for the JTDDataType
returns need to have their first parameter extend never, so that it's
only inferred (or it could be specified manually with `compile<never,
ActualType>(...)`.

In addition, SomeJTDSchemaType needs to use the empty type `{}`, there's
a note linking to the issue that discusses how this is the one instance
when this is actually what you want, and it's tested.

Finally, this works with typescript 4.2.3. However, even with the infer
trick, it's very close to the maximum complexity that typescript wants
to deal with. Small changes in the overload signature or the way that
typescript descides to handle this could result in compile erroring in
typescript saying the type is too complex. In that event, this overload
could always be removed, but I wanted to raise this potential risk now.

fixes ajv-validator#1489