Better types for opcodes

ops.ts exports the list of opcodes as:
```ts
const OPS: { [key: string]: number }
```
That's not a very safe type, as it permits access with any string key. It also doesn't work with code completion.

Can this type be made more specific? It would technically be a breaking change for anyone using this in an unsafe manner, but I believe it's worth it. I'm happy to work on this.