[Crocus] crocus example does not work

Hi, I'm running the examples of crocus (`cranelift/isle/veri/veri_engine`).
The doc says I can test crocus using the following command:

``` bash

cargo build --package=veri_engine # build crocus

path/to/pcrocus -i cranelift/isle/veri/veri_engine/examples/mid-end/broken_bor_band_consts.isle -t simplify --codegen cranelift/codegen/ --x64
```

(I added `--x64` part since the error message tells me to do so)

But it fails with the following error messages

```
Writing generated file: /home/llfuzz/transopt-experiment/wasmtime/output/clif_opt.isle
Writing generated file: /home/llfuzz/transopt-experiment/wasmtime/output/clif_lower.isle
Missing term type instantiation for simplify
```

But the input ISLE file does have instantiation for simplify.

```
// wasmtime/cranelift/isle/veri/veri_engine/examples/mid-end/broken_bor_band_consts.isle
// ...

(spec (simplify x) (provide (= x result)))
(instantiate simplify
((args (bv 8)) (ret (bv 8)) (canon (bv 8)))
((args (bv 16)) (ret (bv 16)) (canon (bv 16)))
((args (bv 32)) (ret (bv 32)) (canon (bv 32)))
((args (bv 64)) (ret (bv 64)) (canon (bv 64)))
)
(decl simplify (Value) Value)

// ...
```

I think the parsing part is not working for crocus.