Added support for freeing variables in the ASR symbolic pass

[anutosh491]

The framework here is simple

1. Anytime we realize that basic_new_stack would be required to allocate memory , that's enough information to confirm that the corresponding variable would also have to be freed at the end of the program. Hence I've declared basic_free_stack accordingly.
2. We run a loop through symbolic_vars which has all CPtr symbols that have been added to the function's symbol table.
3. We can loop through it and keep adding subroutine calls to the function body of main0()

[anutosh491]

I have a doubt . I'm seeing some cases work, for example

def main():
   x: S = pi

Through the pass I can see this block

                                    (SubroutineCall
                                        2 basic_free_stack
                                        2 basic_free_stack
                                        [((Var 3 x))]
                                        ()
                                    )

But when I have

def main():
    print(Symbol("y"))

I get the following error

(lf) anutosh491@spbhat68:~/lpython/lpython$ lpython --show-asr --pass=symbolic examples/expr2.py 
ASR verify pass error: ASR verify: The symbol table was not found in the scope of `symtab`.

ASR verify pass error: ASR verify: Var::m_v `stack0` cannot point outside of its symbol table
Internal Compiler Error: Unhandled exception
Traceback (most recent call last):
  Binary file "/home/anutosh491/lpython/lpython/src/bin/lpython", in _start()
  Binary file "/lib/x86_64-linux-gnu/libc.so.6", in __libc_start_main()
  File "/home/anutosh491/lpython/lpython/src/bin/lpython.cpp", line 1797, in ??
    return emit_asr(arg_file, lpython_pass_manager, runtime_library_dir,
  File "/home/anutosh491/lpython/lpython/src/bin/lpython.cpp", line 252, in ??
    pass_manager.apply_passes(al, asr, pass_options, diagnostics);
  File "/home/anutosh491/lpython/lpython/src/libasr/pass/pass_manager.h", line 280, in LCompilers::PassManager::apply_passes(Allocator&, LCompilers::ASR::TranslationUnit_t*, LCompilers::PassOptions&, LCompilers::diag::Diagnostics&)
    apply_passes(al, asr, _user_defined_passes, pass_options,
LCompilersException: Verify failed in the pass: symbolic

cc @Thirumalai-Shaktivel , I did some debugging but not exactly sure where I might be going wrong .

[Thirumalai-Shaktivel]

Here is the issue for the ASR verify failure:

ASR

symbol_set:
   (Function
      (SymbolTable
            8
            {
               s:
                  (Variable
                        8
                        s
                        []
                        In
                        ()
                        ()
                        Default
                        (Character 1 -2 ())
                        ()
                        BindC
                        Public
                        Required
                        .true.
                  ),
               x:
                  (Variable
                        8
                        x
                        []
                        In
                        ()
                        ()
                        Default
                        (CPtr)
                        ()
                        BindC
                        Public
                        Required
                        .true.
                  )
            })
      symbol_set
      (FunctionType
            [(CPtr)
            (Character 1 -2 ())]
            ()
            BindC
            Interface
            "symbol_set"
            .false.
            .false.
            .false.
            .false.
            .false.
            []
            .false.
      )
      []
      [(Var 8 x)
      (Var 8 s)]
      [(SubroutineCall 						<---------------------
            2 basic_free_stack
            2 basic_free_stack
            [((Var 3 x))]
            ()
      )]
      ()
      Public
      .false.
      .false.
      "symengine/cwrapper.h"
   )

[Thirumalai-Shaktivel]

I left one suggestion, otherwise this LGTM!

[anutosh491]

I've addressed the suggested change , should be ready now.

[certik]

Thanks! I think it looks good. I rebased it to make sure it still works and I set it to merge.