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.

components/script/Cargo.toml

@@ -13,7 +13,7 @@ path = "lib.rs"
 
 [features]
 debugmozjs = ['js/debugmozjs']
-unstable = []
+unstable = ["servo_allocator/unstable"]
 
 [build-dependencies]
 cmake = "0.1"
@@ -77,6 +77,7 @@ script_plugins = {path = "../script_plugins"}
 script_traits = {path = "../script_traits"}
 selectors = { path = "../selectors" }
 serde = "1.0"
+servo_allocator = {path = "../allocator"}
 servo_arc = {path = "../servo_arc"}
 servo_atoms = {path = "../atoms"}
 servo_config = {path = "../config"}