Merge Function/Subroutine into Function

[certik]

Even in ASR the two are slowly diverging, and they should not. I would do something like this:

--- a/src/libasr/ASR.asdl
+++ b/src/libasr/ASR.asdl
@@ -84,12 +84,9 @@ symbol
         stmt* body)
     | Module(symbol_table symtab, identifier name, identifier* dependencies,
         bool loaded_from_mod, bool intrinsic)
-    | Subroutine(symbol_table symtab, identifier name, expr* args, stmt* body,
-        abi abi, access access, deftype deftype, string? bindc_name, bool pure,
-        bool module)
     | Function(symbol_table symtab, identifier name, expr* args, stmt* body,
-        expr return_var, abi abi, access access, deftype deftype, bool elemental,
-        string? bindc_name)
+        expr? return_var, abi abi, access access, deftype deftype,
+        string? bindc_name, bool pure, bool elemental, string? bindc_name)
     | GenericProcedure(symbol_table parent_symtab, identifier name,
         symbol* procs, access access)
     | CustomOperator(symbol_table parent_symtab, identifier name,

If return_var is null, it's a subroutine, otherwise it's a function.

We have written a lot of code that is either exactly identical for both subroutine/function or almost identical. This change would greatly simplify the whole usage.

[certik]

Done!! LFortran and LPython now use just Function.