Leverage fastutil in Index.getTransitiveDeps().
Leverage fastutil in Index.getTransitiveDeps().

Summary:
Previously, `Index.getTransitiveDeps()` took 34% of my standard large
`rdeps()` query; now it takes only 28% of the time.

Reviewed By: sbalabanov

shipit-source-id: 56fb9dc3e6

JoelMarcey/buck,JoelMarcey/buck,facebook/buck,Addepar/buck,kageiit/buck,kageiit/buck,Addepar/buck,facebook/buck,JoelMarcey/buck,JoelMarcey/buck,nguyentruongtho/buck,zpao/buck,JoelMarcey/buck,nguyentruongtho/buck,Addepar/buck,JoelMarcey/buck,Addepar/buck,JoelMarcey/buck,Addepar/buck,JoelMarcey/buck,facebook/buck,facebook/buck,JoelMarcey/buck,JoelMarcey/buck,facebook/buck,nguyentruongtho/buck,zpao/buck,zpao/buck,Addepar/buck,kageiit/buck,kageiit/buck,Addepar/buck,Addepar/buck,Addepar/buck,JoelMarcey/buck,kageiit/buck,kageiit/buck,zpao/buck,JoelMarcey/buck,zpao/buck,nguyentruongtho/buck,Addepar/buck,Addepar/buck,kageiit/buck,JoelMarcey/buck,nguyentruongtho/buck,zpao/buck,nguyentruongtho/buck,nguyentruongtho/buck,facebook/buck,Addepar/buck,Addepar/buck,facebook/buck,zpao/buck