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/tuple.rs

@@ -0,0 +1,45 @@
+use super::{FromRequest, RequestParts};
+use crate::response::IntoResponse;
+use async_trait::async_trait;
+use http::Response;
+use std::convert::Infallible;
+
+#[async_trait]
+impl<B> FromRequest<B> for ()
+where
+    B: Send,
+{
+    type Rejection = Infallible;
+
+    async fn from_request(_: &mut RequestParts<B>) -> Result<(), Self::Rejection> {
+        Ok(())
+    }
+}
+
+macro_rules! impl_from_request {
+    () => {
+    };
+
+    ( $head:ident, $($tail:ident),* $(,)? ) => {
+        #[async_trait]
+        #[allow(non_snake_case)]
+        impl<B, $head, $($tail,)*> FromRequest<B> for ($head, $($tail,)*)
+        where
+            $head: FromRequest<B> + Send,
+            $( $tail: FromRequest<B> + Send, )*
+            B: Send,
+        {
+            type Rejection = Response<crate::body::Body>;
+
+            async fn from_request(req: &mut RequestParts<B>) -> Result<Self, Self::Rejection> {
+                let $head = $head::from_request(req).await.map_err(IntoResponse::into_response)?;
+                $( let $tail = $tail::from_request(req).await.map_err(IntoResponse::into_response)?; )*
+                Ok(($head, $($tail,)*))
+            }
+        }
+
+        impl_from_request!($($tail,)*);
+    };
+}
+
+impl_from_request!(T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16);