Add fs::symlink #684

Job	Run time
test (x86_64-unknown-linux-gnu, stable)	5m 52s
test (x86_64-unknown-linux-gnu, nightly)	14s
test (aarch64-unknown-linux-gnu, stable)	2m 16s
test (aarch64-unknown-linux-gnu, nightly)	15s
test (armv7-unknown-linux-gnueabihf, stable)	4m 23s
test (armv7-unknown-linux-gnueabihf, nightly)	21s
test (riscv64gc-unknown-linux-gnu, stable)	2m 10s
test (riscv64gc-unknown-linux-gnu, nightly)	15s
test (s390x-unknown-linux-gnu, stable)	2m 10s
test (s390x-unknown-linux-gnu, nightly)	15s
test (x86_64-apple-darwin, stable)	4m 23s
test (x86_64-apple-darwin, nightly)	19s
test (aarch64-apple-darwin, stable)	3m 12s
test (aarch64-apple-darwin, nightly)	16s
test (x86_64-pc-windows-gnu, stable)	7m 15s
test (x86_64-pc-windows-gnu, nightly)	33s
test (x86_64-pc-windows-msvc, stable)	6m 9s
test (x86_64-pc-windows-msvc, nightly)	37s
test (i686-pc-windows-gnu, stable)	9m 10s
test (i686-pc-windows-gnu, nightly)	47s
test (i686-pc-windows-msvc, stable)	4m 45s
test (i686-pc-windows-msvc, nightly)	49s
	56m 26s

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