introduce LimitedVec type abstraction

[Stebalien]

Unlike the go actors, we don't enforce maximum sizes in CBOR. Specifically:

• We enforce maximum sizes for bitfields on decode.
• We enforce a maximums size for BigInt.
• We don't enforce maximum byte-buffer/string sizes.
• We don't enforce maximum array sizes.

We need to make sure that the actors have logical checks to ensure that we can't accidentally create state objects that cause problems for, e.g., cron.

Ideally, we'd do this with some length-limited helper types. E.g.:

struct LimitedVec<T, const LIMIT: usize> { ... }

[Stebalien]

Specific concerns:

1. Do we have unbounded inputs? In most cases, this isn't a problem as compute gas will handle it for us. But we should still audit this.
2. Do we have potentially unbounded state. This is the bigger issue.

[anorth]

@Stebalien this was in nv16 milestone. Is it still relevant? If so, when?

[Stebalien]

We audited this and didn't find any issues. However, it would still be nice to introduce some kind of LimitedVec type abstraction just to be extra safe.

On the other hand, this is hardly a priority as we have quite a few size checks (e.g., when setting miner info, etc.). This would just make it easier to reason about those sizes (and allow us to remove the explicit checks).

[anorth]

From #499, also constants like max_aggregated_sectors pre_commit_sector_batch_max_size declarations_max

[Stebalien]

So, once we get proper memory limits:

• We still care about max aggregated sectors because the syscall can only handle so many.
• We don't care about pre-commit batch max size.
• We don't care about declarations max.