WIP: Experimental fpm build

[rouson]

coverage on master

Summary of changes

1. Greatly restructure/simplify the directory structure and remove unsupported legacy files, e.g., GASNet-1 layer.
2. Add fpm manifist: fpm.toml.
3. Add run-fpm.sh script for building with fpm.

To Do

• [ ] Reconcile the file system restructuring with the windows-support branch.
• [ ] Replace the install.sh with an installer based the [Caffeine] installer and the new run-fpm.sh script.

Rationale for changes

1. The Fortran Package Manager has grown greatly in popularity and an fpm manifest so simple to maintain that it's reasonable to support two build systems.
2. Homebrew has become sufficiently stable on macOS and Linux that it will be much easier to maintain a homebred-based installer than the current homegrown-package-management installer.

Additional info and certifications

This pull request (PR) is a:

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

I certify that

• [ ] 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

Code coverage data