Reorganize & rename subdirectories, remove dead code, and document archival code

[rouson]

coverage on master

Summary of changes

This PR

1. Reorganizes and renames subdirectories to clarify the categories of code stored in each subdirectory.
2. Adds src/README.md to clarify that MPI is the only supported communication library and to explain the historical reasons for keeping the unsupported GASNet and OpenSHMEM runtimes.
3. Removes many of the unmaintained static Makefiles, including those that were used to build the unsupported runtimes.
4. Removes dead code in the opencoarrays.F90 module, retaining only the code that is useful for archival purposes because it is used in a publication at https://doi.org/10.1145/3144779.3169110.
5. Updates the build system where necessitated by the above edits.
6. Removes the files that contained the initial implementation for the Fortran 2018 ISO_Fortran_binding.h C header, which has now been incorporated into gfortran and released.

Rationale for changes

We had a lot of unused and unmaintained code associated with projects that dead-ended, e.g., efforts to support communication libraries other than MPI and compilers other than gfortran.

Additional info and certifications

This pull request (PR) is a:

• [ ] Bug fix
• [ ] Feature addition
• [x] Other, Please describe: refactoring

I certify that

• [X] I certify that:
  • I have reviewed and followed the contributing guidelines
  • I will wait at least 24 hours before self-approving the PR to give another OpenCoarrays developer a chance to review my proposed code
  • I have not introduced errant white space (no trailing white space or white space errors may be introduced)
  • I have added an explanation of what these changes do and why they should be included
  • I have checked to ensure there aren't other open Pull Requests for the same change
  • I have you written new tests for these changes
  • I have successfully tested these changes locally
  • I have commented any non-trivial, non-obvious code changes
  • The commits are logically atomic, self consistent and coherent
  • The commit messages follow best practices
  • Test coverage is maintained or increased after this is merged

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