[Typer] Case handling

If we are not planning on supporting nested case I think we can greatly simplify the Pexp.Pcase => Lexp.Case code by replacing

`Pcase of location * pexp * (ppat * pexp) list` by

`Pcase of location * pexp * (ppat list * pexp) SMap.t

with

and ppat = | Ppatany of location | Ppatsym of pvar

The current implementation can support nested patterns (but dont). The new one cant. Nevertheless, the new approach would prevent us from seeing a constructor as a pexp. I am not sure if we lose in generality that much and I think the code will be greatly reduced.