Re-Implement contours with 'lean'.

[julialongtin]

during layer slicing, check the lean of the triangles, and add this data to the Contours such that each segment keeps the triangle's angle to the slice plane. use this data to vary speed, and part-cooling-fan instructions.

[richfelker]

Adding some notes based on IRC conversation: This comes from a more comprehensive slicing design I have in mind, where you can do a lot more with the "lean". In particular, it allows inset for the shell to be computed via a (modified) straight skeleton in 2D while guaranteeing a fixed 3D shell thickness, so that shell thickness (and thereby part strength) doesn't vary with wall steepness like it does with most slicers. It also potentially lets you do some degree of nonplanar printing within layer slices.