Add a task spawning benchmark

This is the kind of workload that the test runner generates - lots of tiny
little tasks - and currently it leaves the CPU underutilized.

src/test/bench/task-perf-spawnalot.rs

@@ -0,0 +1,29 @@
+use std;
+import std::vec;
+import std::task;
+import std::uint;
+import std::str;
+
+fn f(n: uint) {
+    let i = 0u;
+    while i < n {
+        task::join(spawn g());
+        i += 1u;
+    }
+}
+
+fn g() {}
+
+fn main(args: vec[str]) {
+
+    let n = if vec::len(args) < 2u {
+        10u
+    } else {
+        uint::parse_buf(str::bytes(args.(1)), 10u)
+    };
+    let i = 0u;
+    while i < n {
+        spawn f(n);
+        i += 1u;
+    }
+}