Selecting the default global allocator for Servo, and exposing common allocator introspection APIs for memory profiling.
BAO PATCH (embed): only install #[global_allocator] when feature
install-global-allocator is enabled. Library embeds (e.g. frog tools
path-dep on package bao) already provide their own process allocator
(jemalloc / system); dual #[global_allocator] fails the link.