Rewrite code to an older Fortran standard

Once code is parsed into an ASR, be able to rewrite this ASR into a form that can be understood by other Fortran compilers, and then generate Fortran source code. That will allow to use the latest Fortran features (and even LFortran specific extensions such as the custom use extension #44) with any other Fortran compiler.

This is a similar approach that TypeScript uses, it is able to generate JavaScript that works in all browsers.