Stop relying on linking details of std’s default allocator

We’ve been bitten before by symbol names changing:
https://github.com/servo/heapsize/pull/46
and upstream is planning to stop using jemalloc by default:
https://github.com/rust-lang/rust/issues/33082#issuecomment-309781465

So use the (relatively) new `#[global_allocator]` attribute
to explicitly select the system allocator on Windows
and jemalloc (now in an external crate) on other platforms.
This choice matches current defaults.

tests/unit/profile/Cargo.toml

@@ -9,7 +9,13 @@ name = "profile_tests"
 path = "lib.rs"
 doctest = false
 
+[features]
+unstable = ["servo_allocator/unstable"]
+
 [dependencies]
 ipc-channel = "0.9"
 profile = {path = "../../../components/profile"}
 profile_traits = {path = "../../../components/profile_traits"}
+
+# Work around https://github.com/alexcrichton/jemallocator/issues/19
+servo_allocator = {path = "../../../components/allocator"}

tests/unit/profile/lib.rs

@@ -5,6 +5,7 @@
 extern crate ipc_channel;
 extern crate profile;
 extern crate profile_traits;
+extern crate servo_allocator;
 
 #[cfg(test)]
 mod time;