system: is_directory

[perazz]

This PR is spun off #904 for clarity.

• It provides a logical function to evaluate whether a given path is a directory.
• On Windows, it natively works both with / and \ separators.
• Relies on C functions, to avoid Fortran pre-processor macros and path swapping, that would be slow.

Some questions to discuss that come to my mind:

• Should this be part of stdlib_system, stdlib_io, or maybe a submodule(stdlib_system) stdlib_system_filesystem? or else?
• Should we begin standardizing the C-side names somehow? They use the same global namespace, so we should start being careful imho.

cc: @jvdp1 @jalvesz

[perazz]

@jalvesz @jvdp1 @fortran-lang/stdlib mind if I bring your attention to this short PR? If we could get to handle it soon, I would be able to port more of the more complex filesystem operations to stdlib.

[jvdp1]

LGTM. Thank you @perazz

[jalvesz]

LGTM @perazz! Thank for this nice PR!!

[perazz]

Thank you @jvdp1 @jalvesz, let's wait a bit more, and then we can merge imho.

[perazz]

Merging.