Use smallvec to avoid heap allocations

We should use [smallvec](https://github.com/servo/rust-smallvec) for serializing/deserializing as it has the ability to use the stack for small vector allocations.

Noticed that [orjson is doing the same](https://github.com/ijl/orjson/blob/8eea4a301d9b8d8fe726fb3bde026b30ef538257/src/decode.rs#L157-L170).