Break up extract.rs (#103)

This breaks up `extract.rs` into several smaller submodules. The public
API remains the same.

This is done in prep for adding more tests to extractors which would get
messy if they were all in the same file.

src/extract/raw_query.rs

@@ -0,0 +1,39 @@
+use super::{FromRequest, RequestParts};
+use async_trait::async_trait;
+use std::convert::Infallible;
+
+/// Extractor that extracts the raw query string, without parsing it.
+///
+/// # Example
+///
+/// ```rust,no_run
+/// use axum::prelude::*;
+/// use futures::StreamExt;
+///
+/// async fn handler(extract::RawQuery(query): extract::RawQuery) {
+///     // ...
+/// }
+///
+/// let app = route("/users", get(handler));
+/// # async {
+/// # hyper::Server::bind(&"".parse().unwrap()).serve(app.into_make_service()).await.unwrap();
+/// # };
+/// ```
+#[derive(Debug)]
+pub struct RawQuery(pub Option<String>);
+
+#[async_trait]
+impl<B> FromRequest<B> for RawQuery
+where
+    B: Send,
+{
+    type Rejection = Infallible;
+
+    async fn from_request(req: &mut RequestParts<B>) -> Result<Self, Self::Rejection> {
+        let query = req
+            .uri()
+            .and_then(|uri| uri.query())
+            .map(|query| query.to_string());
+        Ok(Self(query))
+    }
+}