Automatically handle http_body::LengthLimitError (#1048)

* Automatically handle `http_body::LengthLimitError`

* add tower-http dev dep to axum-core

* just make it a link

* Make `FailedToBufferBody` an enum

* Fix tests now that tower-http handles `Content-Length`

* Bring back explanation for `LengthLimitError`

* remove todo we likely can't fix

* improve wording in docs

axum-core/src/macros.rs

@@ -10,6 +10,7 @@ macro_rules! define_rejection {
         pub struct $name(pub(crate) crate::Error);
 
         impl $name {
+            #[allow(dead_code)]
             pub(crate) fn from_err<E>(err: E) -> Self
             where
                 E: Into<crate::BoxError>,