Add Fortran backend about lfortran HOT 3 OPEN

Yes, as we discussed, we should go with 2., since it will also be easier to write and maintain, similar to the C backend, but even simpler, since ASR maps better to Fortran.

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TODO

• Visit symbols in Module symtab
• Create a separate function or macro to visit symbols in symtab and handling body for the Nodes like Function, Module, ...
• Handle precedence in Binop and Compare
• Use an enum instead of number in precedence
• Add Fortran to all the tests in the integration_tests CMakeLists (this would fix all the above)
• Add compiler options to decide how some features should be generated (such as generics: either as the extended Fortran keeping all semantics but not possible to compile with gfortran, or lowered to standard fortran, thus losing semantics, but being able to compile with gfortran)
• Add color to the source code (this is low priority, we don't have color in the C backend and perhaps we don't need it --- let's revisit this after all of the above points are fixed and the backend is done)

from lfortran.

I had two ideas:

1. Convert ASR to AST and use ast_to_src() to print the Fortran Source code.
2. Convert ASR directly to source code.

I lean more towards option 2, as it can be used by other frontends as well.

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