Move FromRequest and IntoResponse into new axum-core crate (#564)

* Move `IntoResponse` to axum-core

* Move `FromRequest` to axum-core

* some clean up

* Remove hyper dependency from axum-core

* Fix docs reference

* Use default

* Update changelog

* Remove mention of default type

axum-core/src/error.rs

@@ -0,0 +1,39 @@
+use crate::BoxError;
+use std::{error::Error as StdError, fmt};
+
+/// Errors that can happen when using axum.
+#[derive(Debug)]
+pub struct Error {
+    inner: BoxError,
+}
+
+impl Error {
+    /// Create a new `Error` from a boxable error.
+    pub fn new(error: impl Into<BoxError>) -> Self {
+        Self {
+            inner: error.into(),
+        }
+    }
+
+    pub(crate) fn downcast<T>(self) -> Result<T, Self>
+    where
+        T: StdError + 'static,
+    {
+        match self.inner.downcast::<T>() {
+            Ok(t) => Ok(*t),
+            Err(err) => Err(*err.downcast().unwrap()),
+        }
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.inner.fmt(f)
+    }
+}
+
+impl StdError for Error {
+    fn source(&self) -> Option<&(dyn StdError + 'static)> {
+        Some(&*self.inner)
+    }
+}