Make bin guides compatible with discrete scales

[thomasp85]

This PR makes the new bin guides work with discrete scales if the breaks are given in a cut() compliant format (e.g. (4, 19]).

This, most importantly, makes the bin guides work with geom_contour_filled() but also gives a venue for users to pre-bin their data and still benefit from the new guides.

Example:

library(ggplot2)

volcano_long <- data.frame(
  x = as.vector(col(volcano)),
  y = as.vector(row(volcano)),
  z = as.vector(volcano)
)

ggplot(volcano_long, aes(x, y, z = z)) + 
  geom_contour_filled(aes(fill = stat(level)), bins = 8) + 
  scale_fill_brewer()

ggplot(volcano_long, aes(x, y, z = z)) + 
  geom_contour_filled(aes(fill = stat(level)), bins = 8) + 
  scale_fill_brewer(guide = guide_colorsteps())

^{Created on 2020-01-08 by the reprex package (v0.3.0)}

[thomasp85]

Failure is just standard travis random failure

[clauswilke]

The code looks good to me, but I think adding a few comments would be helpful. Also, should the feature of discrete breaks be mentioned somewhere in the docs?

[clauswilke]

The else branch is sufficiently complicated that a comments would be helpful. In particular, explain that when breaks is not numeric it's expected to be in the (<lower>, <upper>] format. I was very confused by the regex until I figured that out. Also, should this format be mentioned somewhere in the documentation?

[clauswilke]

See my previous comment. Add some explanation to the code here as well.

[thomasp85]

You are probably right about everything. I'll add some comments and improve the docs