Replace dirs-first with dir-order

[xiuxiu62]

This replaces dirs-first with a dir-order option, for ordering directory nodes in branch arms before or after other inodes. It makes some simplifications to how comparators are generated and applied to sort passes on inodes.

Sorry for jumbling this up with another pr. Let me know if you'd like me to make any updates.

[solidiquis]

Hey @xiuxiu62 thanks for taking an interest! Unfortunately I've decided to move forward with #67 mainly because your change would break backwards compatibility, therefore necessitating either a major version bump or a deprecation process leading to an eventual major version bump. I do, however, like the simplification of how comparators are treated so I'd be happy to have that as a separate PR. Let me know your thoughts.

[xiuxiu62]

Completely understand, no worries. I'm not sure it makes sense to change the comparator implementation, unless there are more client options that can be used to produce a comparator. It got a little hairy dealing with the move semantics of multiple comparators when you have more than one branching source, but with just the SortType and dirs-first bool, the original implementation works great. I'd be happy to work on a separate PR though, if it would be beneficial for the project moving forward.