luarocks: manage parallel rocktrees for lua, lua51 and luajit.

Unfortunately github is also showing the #20679, #20680, #20681 and #20705 changesets, since this changeset is predicated upon those. Merging them to master in the correct order will do the right thing though, AFAICT.

I would have preferred to generate the wrapper-scripts (and missing 5.2 .pc file) from associated gist patches, but homebrew appears to have a bug (feature?) where it ignores patch hunks that create a new file, so I had to clutter up the Formula instead.

This essentially finishes the series of pull-requests from my personal branch, and successfully enables full parallel installation of all three common interpreters (lua-5.1, lua-5.2 and luajit) fully integrated with luarocks which manages parallel rocktrees for each. There are further implementation notes in the new changeset itself: https://github.com/gvvaughan/homebrew/commit/c8188a415f00cee39c36b158a5377bd07d5431b5.

If you prefer to relegate 5.2 back to the versions tap, and promote lua51.rb back to lua.rb, everything still works correctly with whatever combination of Lua interpreters and/or luarocks are installed.