Implement submodules

[certik]

Original issue: https://gitlab.com/lfortran/lfortran/-/issues/694

Current status: they parse to ASR, but as separate modules, and then they compile to LLVM as unrelated functionality, so the interface in the parent module does not have an implementation and fails at linking. A simple test was added in https://gitlab.com/lfortran/lfortran/-/merge_requests/1736:

$ lfortran submodule_03.f90
Undefined symbols for architecture arm64:
  "___module_submodule_03_mod_f", referenced from:
      _main in a.out.tmp.o
ld: symbol(s) not found for architecture arm64

[AnshMehta1]

Features of Submodule :-

1. Provides implementation to interface subroutines/functions of parent module. (Should use same program_unit_t nodes)

2. Interfaces are not needed in submodules — only implementations are written there

3. Can access all the entities (public/private) of Parent Module.

4. Functions/subroutines present in Submodule can only be used if its present in Module interface. (We won't get compilation error if module interface doesn't have function/subroutine present in Submodule).

5. Parent module can compile without submodule but using its subroutine or function in interface will cause the error.

6. Submodule should always compile after the compilation of parent module as submodule uses modfile (.mod) of parent module for compilation.

[certik]

For monolithic compilation when we load a module at a use statement in the main program (and recursively) we need to also load any submodules. Some paths forward:

• For each module load we scan the include paths for submodule .mod files (we can even use .smod extension to make it easier) and use the parent module name to find those and load those
• We store the information for each parent module which submodules it has, either by modifying the parent module .mod file or storing this separately in a file somehow.
• The user can tell us explicitly when compiling the main program that there is a submodule, for example by adding the submodule.mod file on the command line; then we simply load it. In cmake build system we'll have to add these extra options when compiling the main program, whether listing the .mod file or by dedicated option like --add-submodule my_submodule.mod.

I like the last option the most. We can start with that, and later on we can investigate if some combination of the previous two options could be better.