regent: Add launcher interface to standard library

This MR adds a new Regent standard library which makes it easier to deploy certain kinds of Regent applications involving a Regent program with an optional mapper. There are two main API calls:

local cmapper = launcher.build_library("mapper")

Builds libmapper.so from mapper.cc. This automatically reads CXX and CXXFLAGS environment variables to use the correct C++ compiler settings for the application.

launcher.launch(main, "app", cmapper.register_mappers, {"-lmapper"})

Either runs the application directly (when SAVEOBJ=0), or builds app and links it to libmapper.so (when SAVEOBJ=1). This also obeys the de facto standard Regent environment variables SAVEOBJ, STANDALONE and OBJNAME.