Add `Deserializer::new` to crate a new deserializer, and public deserializer.

[huacnlee]

We use serde_qs for a custom DeserializeSeed.

ValueDeserializer is my Deserializer.

let input = "title=Hello";
// ValueDeserializer is a custom DeserializeSeed
let deserializer = ValueDeserializer::new();
let de = serde_qs::Deserializer::new(input.as_bytes()).expect("parse QueryString failed");
let value = deserializer.deserialize(de).expect("parse QueryString failed");

This PR just make QsDeserializer pub as Deserializer and add a new method to create a Deserializer.

Like serde_yaml and serde_json:

• https://docs.rs/serde_yaml/latest/serde_yaml/struct.Deserializer.html
• https://docs.rs/serde_json/latest/serde_json/struct.Deserializer.html#method.from_str

[samscott89]

Seems reasonable, thanks!