refactor: rework inspector to take shared references and fix unsoundness (#1853)

Previously we were storing pointers to the raw values, which V8 was holding onto. We didn't actually enforce they were pinned. So this was trivially unsound:

```
let channel = v8::inspector::ChannelBase::new::<ChannelThing>();
let mut channel = ChannelThing { channel };
let client_trust_level = v8::inspector::V8InspectorClientTrustLevel::FullyTrusted;

let session = inspector.connect(
    1,
    &mut channel,
    v8::inspector::StringView::empty(),
    client_trust_level,
);

drop(channel);

// any usage of the session that accesses the channel after this is a UAF
```

We also forced you to embed the base value into your implementation, and all the methods were taking &mut self.

Now, we take shared references (which is ok because we either use an UnsafeCell to get a pointer to hand to V8, or we only use raw pointers)

Author: nathanwhit