Redesign Error type

- The `Error`'s kind is a now a set of variants depending on the context
  of when an error could occur.
- If another error was the cause, it is now always the `source`.

Along with the `is_*` methods, this should help in understanding *when*
a certain error occurred. For example, an error setting the TLS
certificates will return a builder error, with the TLS error as the
source. This should help differentiate from a TLS error that happens
when connecting to a server.

It also makes the internal code less dependent on all the exact
dependencies that can be enabled or disabled.

src/async_impl/body.rs

@@ -154,11 +154,11 @@ impl Stream for ImplStream {
             } => {
                 if let Some(ref mut timeout) = timeout {
                     if let Poll::Ready(()) = Pin::new(timeout).poll(cx) {
-                        return Poll::Ready(Some(Err(crate::error::timedout(None))));
+                        return Poll::Ready(Some(Err(crate::error::body(crate::error::TimedOut))));
                     }
                 }
                 futures_core::ready!(Pin::new(body).poll_data(cx))
-                    .map(|opt_chunk| opt_chunk.map(Into::into).map_err(crate::error::from))
+                    .map(|opt_chunk| opt_chunk.map(Into::into).map_err(crate::error::body))
             }
             Inner::Reusable(ref mut bytes) => {
                 if bytes.is_empty() {