Support for 64-bit integers in JavaScript
I am currently trying to compute a value that is scattered around using an instance, similar to the following example:

```yaml
  something:
    seq:
      - id: a
        type: u1
      - type: b24
      - id: b
        type: u4
    instances:
      fullthing:
        value: '(a<<32) + b'
```

However this doesn't currently work, as the value is assumed to be a 32 bit integer and the calculation overflows. There is no way that I can see to tell it to use 64 bit arithmetic. Allowing "type: u8" or providing a way to "grow" types in this context would come in really handy for this use case.