Rewrite guide_axis() and rename to draw_axis()

[paleolimbot]

This is a PR to prepare for improvements in axis guides as part of #3322. This PR renames the internal function guide_axis() to draw_axis() and includes a complete rewrite of (what is now) draw_axis().

Renaming guide_axis() to draw_axis() is necessary to make way for an exported guide_axis() function that returns a guide (instead of a grob). I used the term "draw" because it is used in many places in ggplot2 for functions that return grobs.

The rewrite of draw_axis() is necessary because the current code is difficult to read and contains much duplication. This makes it difficult to change or improve, as is planned for #3322. The rewrite does not introduce any visual changes in the current tests, and I didn't add any new tests because I couldn't find any part of axis appearance that wasn't already tested between the guides and themes.

[paleolimbot]

I had to modify combine_elements() (in theme.r) to fix the travis build fail for R 3.1 and 3.2. I'm honestly not sure why the builds passed for all the other versions...it looks like calc_element() was never designed to be used for theme "elements" that aren't list() S3s (like tick lengths). @clauswilke I know you're working on theme inheritance, and so if the way I handled this isn't correct let me know. The previous version calculated the tick lengths itself, which seemed redundant to me as the inheritance is defined elsewhere.

[clauswilke]

I'm Ok with the change in combine_elements(). The current way of doing things where code all over the place does it's own inheritance calculations is bad. All of this should be centralized and better formalized.

[hadley]

Need to bring this function up to code, since you've touched. (In this case, that's just restyling) — https://style.tidyverse.org/functions.html#return

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