Remove all extern crate...

In Rust 2018 edition, it is considered unidiomatic to use "extern crate"
sunthax. The proper way to do it is to replace extern crate with "use"
declarations.

- Also impl clippy suggestions for fn main removal in doctests
- Fix unneeded box allocations and clones

Signed-off-by: Damien Stanton <damien.stanton@gmail.com>
Signed-off-by: YUAN LYU <lyuyuan92@gmail.com>

src/server.rs

@@ -397,8 +397,6 @@ impl HttpServer {
     ///
     /// ## Non-blocking server
     /// ```
-    /// extern crate utils;
-    ///
     /// use std::os::unix::io::AsRawFd;
     ///
     /// use micro_http::{HttpServer, Response, StatusCode};