py/emitnative: Initialise locals as Python object type for native code.

In @micropython.native code the types of variables and expressions are
always Python objects, so they can be initialised as such.  This prevents
problems with compiling optimised code like while-loops where a local may
be referenced before it is assigned to.

Signed-off-by: Damien George <damien@micropython.org>

tests/basics/assign_expr.py

@@ -12,6 +12,17 @@ x = 1
 print(x, x := 5, x)
 print(x)
 
+# Test "while" with assignment expression as conditional, assigning to a new local.
+# The while conditional is compiled after the while body, so this tests how the
+# compiler handles the case of an unbound local being compiled before it is assigned.
+def f():
+    l = [0, 1]
+    while local := len(l):
+        print(local, l.pop())
+
+
+f()
+
 
 def foo():
     print("any", any((hit := i) % 5 == 3 and (hit % 2) == 0 for i in range(10)))