FeatureComplete: handle symlinks

[mcharsley]

This involves handling -H, -L and finishing find::matchers::type_matcher.

Take a look at https://github.com/BurntSushi/walkdir/blob/master/examples/walkdir.rs for hints on how to handle -L with walkdir and https://github.com/BurntSushi/walkdir/blob/master/src/tests.rs on how to test interaction with symlinks on both unix and windows machines

[tavianator]

Note that the current behaviour seems to be -H (follow top-level links), not -P (never follow links):

$ ln -s / link
$ find link
link
$ ./target/debug/find link
link
link/var
link/var/cache
link/var/cache/pacman
link/var/cache/pacman/pkg
...