Implicitely inject `extern crate core` #11270

Job	Run time
build-and-check-ubuntu-64bit-glibcxx	31m 1s
build-and-check-gcc-5	32m 54s
build-and-check-ubuntu-64bit	25m 43s
build-and-check-asan	33m 44s
build-and-check-ubuntu-32bit	37m 19s
	2h 40m 41s

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