introduce LimitedVec type abstraction

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.:

```rust
struct LimitedVec { ... }
```