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/connect.rs

@@ -75,7 +75,7 @@ impl Connector {
     where
         T: Into<Option<IpAddr>>,
     {
-        let tls = tls.build().map_err(crate::error::from)?;
+        let tls = tls.build().map_err(crate::error::builder)?;
 
         let mut http = http_connector()?;
         http.set_local_address(local_addr.into());