Replace custom allocator with 'global_allocator'

[jemtucker]

See rust-lang/rust#42727

[jemtucker]

https://github.com/alexcrichton/rust/blob/1389e1c4b713dfc9436c760eb6ffbde806f31c0f/src/doc/unstable-book/src/language-features/global-allocator.md