fastwalk: add ToSlash option for enforcing forward-slash paths

This commit adds the Config.ToSlash option which forces paths to be
joined with a forward-slash instead of the default OS path separator.
This mostly applies to Go binaries that are compiled for Windows but
run under the Windows Subsystem for Linux (WSL) so this commit also
adds a helper function DefaultToSlash which detects if we're a Windows
exe running in WSL.

mod: update go version to 1.20

dirent: handle nil DirEntry in StatDirEntry

gh: test on macos arm64

darwin: cleanup syscall code and use a fixed buffer for opendir

Remove unused syscall code and use a fixed buffer for opendir which
saves us an alloc.

darwin: replace getdirentries64 with readdir_r

The performance benefit of getdirentries64 was negligible to
non-existent and added a huge support burden code wise.