[feature request] lbfs

[lengyijun]

Summary

Support lbfs

Motivation

lbfs is useful in register allocation

Details

• What code needs to change? New traits? Which modules?

Maybe a new module

• Is this a new graph algorithm? Include links to papers, Wikipedia, or other
  implementations of this algorithm that exist.

https://en.wikipedia.org/wiki/Lexicographic_breadth-first_search

Lexicographic Breadth First Search – A Survey

• Are you willing to implement this yourself? Mentor someone else and help them
  implement it?

lbfs uses linked list

Rust stable std does not support linked list, which is a concern

I have a sample implementation depends on nightly rust