Fortran Pathlib

• add %unlink,%drop_sep,%size_bytes, is_exe
• remove spurious warnings
• mkdir: use CRT.

For quality and performance, reimplement internally as plain functions, and expose those functions to users.

Most methods are also exposed as functions

Broke up code into files by high-level e.g. gcc/intel, impure/pure, windows/posix. This help dev clarity. Also make more test source files.