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/response.rs

@@ -227,7 +227,7 @@ impl Response {
     pub async fn json<T: DeserializeOwned>(self) -> crate::Result<T> {
         let full = self.bytes().await?;
 
-        serde_json::from_slice(&full).map_err(crate::error::from)
+        serde_json::from_slice(&full).map_err(crate::error::decode)
     }
 
     /// Get the full response body as `Bytes`.