add os.pathExists; fix RFCs/issues/121

[timotheecour]

do not review yet, I'm still trying to figure whether pathExists or pathKind is the most sensible API to add.

• fixes os module should have an "exists" function RFCs#121
• refactor/simplify existing code to remove code duplication

TODO

• all Xexists procs (including this one) are wrong wrt permission errors, they return false when a file can't be stat'd due to permission error, instead they should either raise, or raise when a check bool flag is provided, or some other behavior
• instead of pathExists, maybe we should have pathKind which could return:

{symLink, file, dir, device, none, permissionError}

note: PathComponent is IMO the wrong thing to return here, it's not what the OS provides directly, doesn't distinguish broken symlinks, doesn't have a category for device files etc

[stale]

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