Add optional enum-ptr feature

Hey, thanks for creating this crate. I'm planning to use it, combined with a crate called `enum-ptr`: https://crates.io/crates/enum_ptr `enum-ptr` provides a marker trait called `Aligned` (https://docs.rs/enum-ptr/latest/enum_ptr/trait.Aligned.html), which `rclite::{Rc, Arc}` needs to implement for our use case. So, this pr is adding optional feature to do so. Along with it, this pr also adjusts the CI so that the newly added code is tested.