A simpler overlay to speed up reference path queries

[jeizenga]

Includes a new overlay, ReferencePathOverlay, that has similar functionality to the PackedReferencePathOverlay except with simpler backing data structures that should be faster to access. The construction algorithm is pretty parallel, so hopefully it won't add too much to start-up time.

I've stolen the name ReferencePathOverlayHelper to refer to the ReferencePathOverlay. The older overlay helper is now named PackedReferencePathOverlayHelper.

@adamnovak This does PR does not include any updates to the Python bindings. I'm not sure if I have easy access to a computational environment where I can run the binder code. Is this mergeable without that?

[adamnovak]

I'm not sure we ever bound the overlay helpers, so there probably isn't a breaking change vs. the existing generated binding code we have checked in.