[R-SIG-Mac] RSwitch 64-bit

[Bob Rudis]

On http://mac.r-project.org/ (which shld prbly be re-titled to "R for macOS Developer's Page"…I'd file a PR for that but do not know where that website source tree is maintained) there's a tiny binary at bottom of the page that enables switching of the `Current` R framework alias via a GUI.

TLDR: It's 32-bit and the source links stopped working even when the site was homed @ AT&T. As a result, it's unusable on the forthcoming Catalina GA release.

I have no idea how popular said tool is, but it's handy, so I threw together a Swift 5 version of it (and set a minimum macOS target of 10.14 since I cannot ethically enable use of woefully insecure operating systems). A core difference between the 2011 version and this 2019 version is that it's a menubar app vs a window app.

The code and binary release are at <https://git.rud.is/hrbrmstr/RSwitch>, which has links to other social coding sites where the codebase is co-maintained.

I have a bit more work to do on it (mostly just rounding out corners) but it has "worked for me" on Catalina betas and any/all contributions are welcome/encouraged.

-boB