Panic in normalizer when checking pathological example

[timsueberkrueb]

Consider the following example (note the missing case for C in type:

data Foo {
    A,
    B(y: C(A).type),
    C(x: B(A).type)
}

def Foo.type: Type {
    A => Foo,
    B(_) => Foo,
}

Typechecking fails with the following panic:

Error:   × Main thread panicked.
  ├─▶ at lang/elaborator/src/normalizer/eval.rs:160:91
  ╰─▶ called `Option::unwrap()` on a `None` value
  help: set the `RUST_BACKTRACE=1` environment variable to display a
        backtrace.