filesystem: error handling, delete_file, is_directory and other

[perazz]

this PR follows up (without overlapping) on the excellent work ongoing at #874.
Also CC: #14 and j3-fortran/fortran_proposals#100
An initial stdlib_io_filesystem functionality is proposed: because such minimal functionality is intertwined, this PR proposes to introduce the following:

• extend state_type error handling, that was well accepted for linear algebra, to the whole library (without API changes: just make state_type->linalg_state_type polymorphism)
• Runtime OS query (saved to static storage for performance)
• Basic file I/O and sub-processing functions: delete_file, is_directory, run, getfile

stdlib_io_filesystem

• is_directory(path): logical function
• call delete_file(path [, err]): optional return state, otherwise exceptions trigger an error stop

stdlib_system

• call run(cmd [, exit_state] [, command_state] [, stdout] [, stderr]): with optional stdout/stderr outputs loaded onto string_type variables
• function os_type() returns a cached OS type flag, OS_NAME its name
• function null_device() returns a cross-platform string path to suppress terminal output

stdlib_io

• type(string_type) function getfile(fileName [,err] [,delete]) loads a whole ASCII file into a string_type variable all at once, with option to return a state flag, or to request file deletion (delete=.true.) after loading. This function is necessary within this PR for loading screen output.

Comments

• Usage of external C code has been avoided, but I'm happy to change implementation details if requested. In fpm, there is a policy that the package must be fully compiled as a single-source Fortran file, so C usage is not well integrated, but I don't know what the stdlib policy is in this respect.
• should delete functionality be unique for either files or directories? otherwise, I will work out a rmdir or delete_dir alternative for them.

cc: @fortran-lang/stdlib @jvdp1 @jalvesz @zoziha @minhqdao