improve performance for advancing front

[shuoli84]

Current impl, advancing front is backed by a quick sorted vector, which starts to cause issue for large graph.

[shuoli84]

cargo flamegraph --example draw --root -- --path ../test/poly2tri/testbed/data/nazca_heron.dat --bench-count 20000

shows about 1/4 of time is spend in advancing front related code.

[shuoli84]

maybe splaytree? it is self optimizing(access the last accessed element will be fast), with fast insert and remove operation.