Implement Pluggable Name-resolution (#148)

This defines a new trait `Resolver`, which turns an address into a
Vec<SocketAddr>. It also provides an implementation of Resolver for
`Fn(&str)` so it's easy to define simple resolvers with a closure.


Fixes #82

Co-authored-by: Ulrik <ulrikm@spotify.com>

src/unit.rs

@@ -10,6 +10,7 @@ use cookie::{Cookie, CookieJar};
 use crate::agent::AgentState;
 use crate::body::{self, Payload, SizedReader};
 use crate::header;
+use crate::resolve::ArcResolver;
 use crate::stream::{self, connect_test, Stream};
 use crate::{Error, Header, Request, Response};
 
@@ -95,6 +96,10 @@ impl Unit {
         self.req.method.eq_ignore_ascii_case("head")
     }
 
+    pub fn resolver(&self) -> ArcResolver {
+        self.req.agent.lock().unwrap().resolver.clone()
+    }
+
     #[cfg(test)]
     pub fn header(&self, name: &str) -> Option<&str> {
         header::get_header(&self.headers, name)