Add Serde Support For AST

[sodiumjoe]

branched from #77

[jondlm]

Any chance we could get this merged/released soon? I understand if y'all are busy. I'll begin the process of forking internally but ideally we can unwind that soon.

[LegNeato]

@tailhook Can you take a look?

[LegNeato]

Not sure if this is a good idea, but I think to not duplicate everything you could do something like:

#[cfg(feature = "serde")]
mod text_traits {
    use serde::{Serialize, Deserialize};

    pub trait SerdeTraits<'a>: Serialize + Deserialize<'a> {}

    impl<'a, T> SerdeTraits<'a> for T where T: Serialize + Deserialize<'a> {}
}

#[cfg(not(feature = "serde"))]
mod text_traits {
    pub trait SerdeTraits<'a> {}

    impl<'a, T> SerdeTraits<'a> for T {}
}

// Define the main trait with conditional compilation
pub trait Text<'a>: 'a {
    type Value: 'a
        + From<&'a str>
        + AsRef<str>
        + std::borrow::Borrow<str>
        + PartialEq
        + Eq
        + PartialOrd
        + Ord
        + fmt::Debug
        + Clone
        + text_traits::SerdeTraits<'a>;
}