Some forum users noticed that `sm.exec()` took longer the more was present
on the flash filesystem connected to the RP2040. They traced this back to
the `array` import inside `asm_pio()`, which is causing MicroPython to scan
the filesystem.
uarray is a built-in module, so importing it shouldn't require scanning the
filesystem.
We avoid moving the import to the top-level in order to keep the namespace
clean; we don't want to accidentally expose `rp2.array`.
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