Move task::task() to TaskBuilder::new()

The constructor for `TaskBuilder` is being changed to an associated
function called `new` for consistency with the rest of the standard
library.

Closes #13666

[breaking-change]

src/test/bench/msgsend-pipes-shared.rs

@@ -23,6 +23,7 @@ extern crate time;
 use std::comm;
 use std::os;
 use std::task;
+use std::task::TaskBuilder;
 use std::uint;
 
 fn move_out<T>(_x: T) {}
@@ -62,7 +63,7 @@ fn run(args: &[~str]) {
     let mut worker_results = Vec::new();
     for _ in range(0u, workers) {
         let to_child = to_child.clone();
-        let mut builder = task::task();
+        let mut builder = TaskBuilder::new();
         worker_results.push(builder.future_result());
         builder.spawn(proc() {
             for _ in range(0u, size / workers) {

src/test/bench/msgsend-pipes.rs

@@ -18,6 +18,7 @@ extern crate time;
 
 use std::os;
 use std::task;
+use std::task::TaskBuilder;
 use std::uint;
 
 fn move_out<T>(_x: T) {}
@@ -56,7 +57,7 @@ fn run(args: &[~str]) {
     let mut worker_results = Vec::new();
     let from_parent = if workers == 1 {
         let (to_child, from_parent) = channel();
-        let mut builder = task::task();
+        let mut builder = TaskBuilder::new();
         worker_results.push(builder.future_result());
         builder.spawn(proc() {
             for _ in range(0u, size / workers) {
@@ -70,7 +71,7 @@ fn run(args: &[~str]) {
         let (to_child, from_parent) = channel();
         for _ in range(0u, workers) {
             let to_child = to_child.clone();
-            let mut builder = task::task();
+            let mut builder = TaskBuilder::new();
             worker_results.push(builder.future_result());
             builder.spawn(proc() {
                 for _ in range(0u, size / workers) {

src/test/bench/shootout-pfib.rs

@@ -24,6 +24,7 @@ extern crate time;
 use std::os;
 use std::result::{Ok, Err};
 use std::task;
+use std::task::TaskBuilder;
 use std::uint;
 
 fn fib(n: int) -> int {
@@ -77,7 +78,7 @@ fn stress_task(id: int) {
 fn stress(num_tasks: int) {
     let mut results = Vec::new();
     for i in range(0, num_tasks) {
-        let mut builder = task::task();
+        let mut builder = TaskBuilder::new();
         results.push(builder.future_result());
         builder.spawn(proc() {
             stress_task(i);