Better breaks calculations for exponential transformations

[thomasp85]

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Superseedes #59

While we no longer see errors in ggplot2 for these transformations (the report in the original issue), the placement of breaks with various transformations still leave a lot to be desired

library(ggplot2)
dat <- data.frame(x = 5:100)

ggplot(dat) + 
  aes(x = x, y = x) + 
  geom_point() +
  scale_y_continuous(trans = "exp")