rp2: Import uarray instead of array in rp2 module.

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`.

ports/rp2/modules/rp2.py

@@ -33,7 +33,9 @@ class PIOASMEmit:
         push_thresh=32,
         pull_thresh=32
     ):
-        from array import array
+        # uarray is a built-in module so importing it here won't require
+        # scanning the filesystem.
+        from uarray import array
 
         self.labels = {}
         execctrl = 0