feat: add rkyv derive for Candle

[harryagstian]

Added rkyv derive for Candle behind feature gate, allowing the struct to be serialized / deserialized / stored using rkyv.

Using rkyv significantly improved candles load performance for files, which is useful when doing backtesting. I am able to load 232k candles in 2 seconds using rkyv, compared to 28 seconds with serde-json.

Also, minor fix on donchian_channel documentation.

[amv-dev]

I don't see any reason to add such a dependency in the crate.

If you need some specific behavior for Candle, you can choose one of at least two ways:

1. Create you own Candle. The only requirement here is to implement OHLCV trait for it and it's pretty simple.
2. Create your own trait which will handle behavior you need and implement it to existing Candle.