Sketch a typed version of core

[b-studios]

I started working on a typed version of our core representation. It would be great to gather feedback on it.

In particular, I am interested in

• learning whether it impedes or helps other backends
• It impedes or helps potential optimization passes
• how to represent includes in core
• how to model top-level / module-level declarations
• how to represent mutually recursive bindings (blocks / data defs / interface defs)

@marzipankaiser @jiribenes @phischu @jfrech Please chip in, if you have any comments.