We’ve been bitten before by symbol names changing:
servo/heapsize#46
and upstream is planning to stop using jemalloc by default:
rust-lang/rust#33082 (comment)

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.