Consolidate coordinate scale expansion and limiting code

[paleolimbot]

This is a PR to address coordinate scale expansion and limiting using a single approach, implementing several feature requests and fixing several bugs in the process (closes #2990, closes #2907, closes #3056, closes #3336). The inspiration for this PR was a collection of issues that @hadley collected when issue triaging that were related to the interface between scales and coordinate systems (#3371). I'd be happy to split this up into a few different PRs if it is easier to review (one for expansion, one for coord limiting, one for sprucing up coord_trans).

• All scale expansion code now lives in scale-expansion.r, and is tested independently of the scales/coords. All Coord objects now use these expansion functions. Previously, some form of scale expansion code lived in utilities.r, scale-.r, coord-sf.r, coord-cartesian.r, scale-discrete.r, and coord-transform.r. This led to at least one issue of inconsistent expansion between coords (Remove extra space created by coord_trans #3338), and was thought to cause several other issues, although it turns out that Secondary axis doesn't show axis ticks or labels with coord_trans #2990 was unrelated, Scale expand value from scales ignored when coord_cartesian ylim is used #3270 had already been fixed before this PR, and nothing about this PR addressed sqrt_trans, scale limit expansion, and missing breaks #980.

• All coord_*() functions with xlim and ylim arguments now accept vectors with NA as a placeholder for the minimum or maximum value (e.g., ylim = c(0, NA) would zoom the y-axis from 0 to the maximum value observed in the data). This mimics the behaviour of the limits argument in continuous scale functions (NA limits in coords #2907).

library(ggplot2)
ggplot(mpg, aes(cty, hwy)) + 
  geom_point() +
  coord_cartesian(xlim = c(20, NA), ylim = c(NA, 40))

• coord_trans() now draws second axes (Secondary axis doesn't show axis ticks or labels with coord_trans #2990) and accepts xlim, ylim, and expand arguments to bring it up to feature parity with coord_cartesian(). This also closes Deprecate limx and limy in coord_trans() #3056, which is an abandoned PR that deprecated limx and limy in favour of xlim and ylim. This PR does the same, and also fully removes xtrans and ytrans, which have been deprecated long enough that using them resulted in an error anyway.

• coord_trans() now calculates breaks using the expanded range (previously these were calculated using the unexpanded range, which resulted in differences between plots made with coord_trans() and those made with coord_cartesian(). The expansion for discrete axes in coord_trans() was also updated such that it behaves identically to that in coord_cartesian() (see Remove extra space created by coord_trans #3338 - this was already closed by the OP, but it is the best discussion of this topic).

library(ggplot2)

p <- ggplot(mpg, aes(class, hwy)) + geom_boxplot() 

# these two plots didn't used to align and sometimes had
# different breaks on the y-axis
p + scale_y_log10()
p + 
  coord_trans(y = "log10") + 
  scale_y_continuous(breaks = scales::log_breaks())

• Added regression tests for coord_trans(), for which previously there were none.

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