Should have serde module for encoding values as strings

[apoelstra]

I don't think we should have a default serde impl for Value since there is no standard way to encode values. But we should provide a serialization method so users can at least write

#[derive(Serialize)]
struct MyString {
    #[serde(serialize_with = "simplicity::serde::stringified_value")]
    inner: Value,
}

or something.

[apoelstra]

I would like this for hal-simplicity.