Allow to "use" modules compiled by gfortran

One can compile production code with gfortran, which produces gfortran specific mod files. Those can already be parsed by lfortran.adapters.gfortran.mod, which creates an equivalent ASR representation of the module.

Now we just need to allow them to be used "out of the box" with LFortran simply by "using" them:

use some_mod, only: some_fun
call some_fun(5)

where some_mod was compiled with gfortran.

Later we can add support for other Fortran compilers also (#56).

This will allow to use LFortran with production codes right away. Simple functions in a module would work, and for more complex functionality not yet implemented by LFortran all one has to do is to write a simple module, compile with gfortran, and then use in LFortran interactively.

Note: a related project is gfort2py, which down the road can hopefully use lfortran.adapters.gfortran.mod so that they do not have to maintain their own parser.