Pre-requisite for #564

Hi Howard,

I am very motivated to add support for using a binary tzdb on Windows. I hope that that is something that you would also be interested in, and would be willing to add support for, even though you are on a feature freeze.

As a first step, I've added a PR to my own R package (that uses <date>) with changes to the tz code to allow for setting the binary tzdb path at runtime. It seems to work pretty well for me with binary files I pulled from CCTZ's github repo. It currently is only allowed for non-Windows platforms, because I have a feeling the binary parser might have to be tweaked for Windows. Nevertheless, this is a required step for Windows support, since I'd have to provide a binary tzdb in a custom location for that platform. It will also be useful for me on non-Windows platforms, as I can standardize on a specific tzdb version across all platforms.

I've documented the changes in the PR, and would love the opportunity to include them as a PR into date if you think they are appropriate. I've tried to be as backwards compatible as possible. The first 3 commits are relevant to you. Ignore the large amount of changed files, those are just the binary zoneinfo files.
r-lib/clock#6

Let me know what you think!