Port all of LFortran to use ASR
Currently the lfortran.codegen.gen uses the old AST+annotations approach. We need to extend lfortran.codegen.asr_to_llvm which converts the ASR to LLVM to be able to handle all the constructs that lfortran.codegen.gen can, and then remove lfortran.codegen.gen.
As part of this, once lfortran.codegen.gen is removed, also remove lfortran.semantic.analyze and only keep lfortran.semantic.ast_to_asr.