This commit
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 necessitateed by the above edits.