CI: error on dead/unused code for features (#146)

By using `RUSTFLAGS="-D dead_code -D unused-variables -D unused"`, we tell
the compiler to upgrade warnings for unused code to errors. This combined
with a more elaborate test matrix means we can have the CI catch feature
flag combos that cause warnings.

src/agent.rs

@@ -282,7 +282,6 @@ pub(crate) fn basic_auth(user: &str, pass: &str) -> String {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use std::io::Read;
     use std::thread;
 
     ///////////////////// AGENT TESTS //////////////////////////////
@@ -298,6 +297,8 @@ mod tests {
     #[test]
     #[cfg(any(feature = "tls", feature = "native-tls"))]
     fn agent_pool() {
+        use std::io::Read;
+
         let agent = crate::agent();
         let url = "https://ureq.s3.eu-central-1.amazonaws.com/sherlock.txt";
         // req 1