feat(neon): Add more TryIntoJs and TryFromJs implementations

[kjvalencik]

While trying #[neon::export] out in another project, I found some trivial TryFromJs and TryIntoJs implementations that simplified code.

• TryFromJs for Root<T>. We can call .root(cx) when extracting
• TryIntoJs for Box<T> and Arc<T> smart pointers. This is especially useful if you want to return an Arc<Vec<T>> and convert it into a typed array.
• TryIntoJs for &Vec<T> and &String. We already had the slice versions, but it's nice to have these as well when combined with the smart pointers above.

I wanted to implement for T where T: AsRef<U> and for<'a> &'a U: TryIntoJs<'cx>, but this isn't possible since U wouldn't be bound by the implementation. It also conflicts with some specialized versions.

There's a bunch more that we could add (e.g., Box<str>), but I'm trying to be a little conservative and not implement for a bunch of strange types that users don't need. These are all convenience implementations anyway. Users have With as an escape hatch.

[dherman]

This is super nice. 🚢 it!