Add FileEntry::linkto for symbolic links #628

Job	Run time
Clippy (stable)	59s
Check (stable)	43s
Rustfmt (stable)	7s
Compatibility Test Suite (stable)	5m 50s
Tarpaulin (stable)	6m 19s
Test Suite (stable)	1m 40s
Test Suite (1.67.0)	7m 14s
Test Suite (feature-matrix) (stable, --all-features)	7m 38s
Test Suite (feature-matrix) (stable, --no-default-features)	1m 32s
Test Suite (feature-matrix) (1.67.0, --all-features)	10m 31s
Test Suite (feature-matrix) (1.67.0, --no-default-features)	6m 12s
	48m 45s

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