Easily convert typed paths into URIs (#790)

* Easily convert typed paths into URIs

`#[derive(TypedPath)]` will now also generate `TryFrom<_> for Uri` for
easily converting paths into URIs for use with `Redirect` and friends.

Fixes https://github.com/tokio-rs/axum/issues/789

* Use a method on the `TypedPath` trait to convert to `Uri`

* fix doc ref

* Update changelogs

axum-macros/tests/typed_path/pass/into_uri.rs

@@ -0,0 +1,23 @@
+use axum_extra::routing::TypedPath;
+use axum::http::Uri;
+use serde::Deserialize;
+
+#[derive(TypedPath, Deserialize)]
+#[typed_path("/:id")]
+struct Named {
+    id: u32,
+}
+
+#[derive(TypedPath, Deserialize)]
+#[typed_path("/:id")]
+struct Unnamed(u32);
+
+#[derive(TypedPath, Deserialize)]
+#[typed_path("/")]
+struct Unit;
+
+fn main() {
+    let _: Uri = Named { id: 1 }.to_uri();
+    let _: Uri = Unnamed(1).to_uri();
+    let _: Uri = Unit.to_uri();
+}