WIP: Add support for heapless::Vec

[nickray]

For embedded use cases, instead of alloc it's often preferable to use heapless: fixed-capacity strings and vectors. This patch would allow serializing Vec<u8> as byte strings instead of generic vectors.

A minimal example: https://github.com/nickray/serde-bytes-heapless-test/blob/master/src/lib.rs

I think it would be nice to have this patch here, would that make sense? The alternative being heapless having a Bytes type itself (rust-embedded/heapless#138).

Regarding tests, I'm not sure how they could be added.

Regarding error handling, I'm not sure if or how to map capacity errors when deserializing instead of horribly panicking.

[dtolnay]

I think I would prefer not to build this into serde_bytes, but it would be reasonable to maintain this in a separate crate. As long as it has:

use heapless::{ArrayLength, Vec};
use serde::{Deserialize, Deserializer, Serialize, Serializer};

pub fn serialize<T, N, S>(vec: &Vec<T, N>, serializer: S) -> Result<S::Ok, S::Error>
where
    T: Serialize,
    N: ArrayLength<T>,
    S: Serializer,
{...}

pub fn deserialize<'de, D, T, N>(deserializer: D) -> Result<Vec<T, N>, D::Error>
where
    D: Deserializer<'de>,
    T: Desererialize<'de>,
    N: ArrayLength<T>,
{...}

that will make #[serde(with = "heapless_serde_bytes")] work.

[nickray]

Understood. I am currently maintaining https://crates.io/crates/heapless-bytes and working towards merging this into heapless proper (rust-embedded/heapless#164).