extract worksheet reading code from functions

[jennybc]

This is not a genuine PR @richfitz. I've created a faux vignette, where I unpack all the code necessary to read the data from a workbook with one sheet. I propose this PR/vignette as a way to have a concrete conversation as I figure out how rexcel works. Eventually we could comment around specific lines. But first, can I get your high-level reaction?

1. Does this seem a decent way to proceed?
2. It feels like rexcel's reading could be made simpler, but of course I don't have specific proposals yet! Is that also your sense or do you think there aren't any major simplifications available?
3. I do still want you to look after Excel, while I look after Google. So I'm trying to not get in too deep here. But I think I have to get some understanding of rexcel, because it's driven the current specs of linen. This is more a comment than question.

[richfitz]

Nice! I have more packages you could do this to if you'd like 😀

readxl's reading could be made simpler, but for my medium term sanity the main thing I'd like to keep is that most of the low-level stuff mirrors the names in the spec (all the functions with ct and st in them). I can comb through and see if it can be cleaned up, but the main thing that I know is needed is some further quarantining of style code.

I'll try and get through the actual vignette soon

[jennybc]

I think quarantining of the style code could be achieved at the same time as my suggestion to create a workbook registration function (that treats all worksheets equally and gathers a bit of info on them), should you choose to accept it.