Implicitely inject `extern crate core` #11272

Job	Run time
build-and-check-asan	33m 53s
build-and-check-ubuntu-32bit	36m 6s
build-and-check-gcc-5	32m 54s
build-and-check-ubuntu-64bit-glibcxx	31m 2s
build-and-check-ubuntu-64bit	24m 50s
	2h 38m 45s

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