Add Dir::{read_link_contents,symlink_contents} methods #1661

Job	Run time
Check Cross-Compilation on Windows (stable)	1m 21s
Check Cross-Compilation (stable)	1m 34s
Check Cross-Compilation on Windows (beta)	1m 15s
Check Cross-Compilation on Rust nightly on Windows (nightly)	1m 38s
Check Cross-Compilation on Rust nightly (nightly)	4m 50s
Check Cross-Compilation (beta)	2m 4s
Rustfmt	9s
Fuzz Targets	1m 35s
Test (stable)	51s
Test (windows-latest)	1m 50s
Test linux-raw support (ubuntu)	34s
Test (msrv)	2m 23s
Test (windows-2019)	2m 10s
Test linux-raw support (ubuntu-20.04)	34s
Test on Musl (ubuntu)	1m 3s
Test (macos-latest)	1m 22s
Test (macos-11)	1m 10s
Test with Rust nightly (ubuntu)	34s
Test (beta)	35s
Test with Rust nightly (windows)	1m 25s
Test (ubuntu-20.04)	29s
Test (aarch64-ubuntu)	3m 11s
	32m 37s

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