Add helper functions constructing OrderedFloat and BigInt to edn crate

[ncalexan]

Right now, real consumers of edn::Value need to import OrderedFloat and BigInt in order to pattern match Value completely. That means to extern crate declarations, which is awkward. This ticket tracks exposing simple constructors in the edn crate. For example, maybe

impl Value {
    from_float(x: f64) -> Value { ... }
    from_bigint(x: &str) -> Value { ... }
}

[victorporof]

I'll have a go at this.