Support matching on chars

[Nadrieril]

This code:

fn main() {
    match 'x' {
        'a' => {}
        _ => {}
    }
}

Causes a panic:

error[E9999]: Unexpected switch_ty: Ty { kind: Node { id: Id { id: 5 }, value: Char } }
  |
  = note: ⚠️ This is a bug in Hax's frontend.
          Please report this error to https://github.com/hacspec/hax/issues with some context (e.g. the current crate)!

error: Hax panicked when translating ...

It seems we never tested matching on chars.