Fix compile error in std::fs impl on VEXos target

[lewisfm]

This PR fixes a compile error in the standard library on the armv7a-vex-v5 target that was caused by there not being a version of the Dir struct exported from std::sys::fs::vexos. Reading from directories isn't supported on this platform, so the module now re-exports the unsupported version of Dir.

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[joboet]

@bors r+ rollup

[rust-bors]

📌 Commit 04f6a49 has been approved by joboet

It is now in the queue for this repository.