Add a "Core" intermediate representation about fathom

[brendanzab]

One idea to get around the error reporting problem is to do what GHC does: implement a type checker+inference on the complex AST and a simple, more elegant type checker on the fully explicit core representation as a sanity check for debugging purposes.

[mikeday]

It's inevitable that we will want to add some helpful syntax that gets desugared in the core representation.

[brendanzab]

Could be as close to a direct port of the typechecker in the formal specification, perhaps, seeing as that would have progress, preservation, and soundness already verified:

https://github.com/yeslogic/ddl/blob/06ffee90a00b119a145babd1f9339fcd960f519c/experiments/lean/ddl.lean#L275-L296

In the future we could also have specifications of how the high level code is 'lowered' into the desugared representation.

[mikeday]

Fun!