pointToIndex mutates the coordinates of the provided point

[Brett Johnson]

Mutation of the point provided as an argument to pointToIndex causes confusing hard to track bugs. It would be helpful to either document the behavior or make the function pure.

See:
https://github.com/mhyfritz/hilbert-curve/blob/master/src/index.js#L59
https://github.com/mhyfritz/hilbert-curve/blob/master/src/index.js#L69