Implicitely inject `extern crate core` #11269

Job	Run time
build-and-check-gcc-5	25m 26s
build-and-check-ubuntu-32bit	23m 20s
build-and-check-ubuntu-64bit	21m 26s
build-and-check-asan	27m 33s
build-and-check-ubuntu-64bit-glibcxx	18m 30s
	1h 56m 15s

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