Start changes to use deb-indices and drop type inference

* src/lexp.ml: Start changes to use deb-indices and simplify things.
(vref): New type.
(lexp): Var now uses debruijn-indices.
Comment out metavars since we're not going to do inference for now.
(type_eq, builtins, lexp_location): Adjust accordingly.
(lexp_alpha): Remove since deb-indices don't need it.
* typer-mode.el (typer-smie-grammar): Add an explicit level for "=".