time: Deprecate the library in the distribution

This commit deprecates the entire libtime library in favor of the
externally-provided libtime in the rust-lang organization. Users of the
`libtime` crate as-is today should add this to their Cargo manifests:

    [dependencies.time]
    git = "https://github.com/rust-lang/time"

To implement this transition, a new function `Duration::span` was added to the
`std::time::Duration` time. This function takes a closure and then returns the
duration of time it took that closure to execute. This interface will likely
improve with `FnOnce` unboxed closures as moving in and out will be a little
easier.

Due to the deprecation of the in-tree crate, this is a:

[breaking-change]

cc #18855, some of the conversions in the `src/test/bench` area may have been a
little nicer with that implemented

src/librustdoc/lib.rs

@@ -25,7 +25,6 @@ extern crate rustc;
 extern crate serialize;
 extern crate syntax;
 extern crate "test" as testing;
-extern crate time;
 #[phase(plugin, link)] extern crate log;
 
 use std::io;
@@ -238,7 +237,6 @@ pub fn main_args(args: &[String]) -> int {
     };
 
     info!("going to format");
-    let started = time::precise_time_ns();
     match matches.opt_str("w").as_ref().map(|s| s.as_slice()) {
         Some("html") | None => {
             match html::render::run(krate, &external_html, output.unwrap_or(Path::new("doc"))) {
@@ -257,8 +255,6 @@ pub fn main_args(args: &[String]) -> int {
             return 1;
         }
     }
-    let ended = time::precise_time_ns();
-    info!("Took {:.03f}s", (ended as f64 - started as f64) / 1e9f64);
 
     return 0;
 }