Use fork of go-queue.

To protect against it going away or breaking backwards compatibility.
Move the point cleaning into the Add() method.

Doesn't affect performance and will reduce memory use in the case of many Add()s before Calculate()s.
Store pointers instead of values for points.

Benchmarking shows from ~163 ns/op down to 130 ns/op in Add()s.
Improve test error messages.
Remove unneeded nil check for front point in Add().
Add test for Add()ing a point with an old x value.
Add test for multiple Calculate()s.
Add build to enable checking for code coverage.