Migrate to op2. Making a few decisions to get this across the line:
- Empty slices, no matter where the come from, are null pointers. The v8
bugs (https://bugs.chromium.org/p/v8/issues/detail?id=13489) and
(https://bugs.chromium.org/p/v8/issues/detail?id=13488) make passing
around zero-length slice pointers too dangerous as they might be
uninitialized or null data.
- Offsets and lengths are `#[number] isize` and `#[number] usize`
respectively -- 53 bits should be enough for anyone
- Pointers are bigints. This is a u64 in the fastcall world, and can
accept Integer/Int32/Number/BigInt v8 types in the slow world.