py: Fix up number operations and coercion.

examples/mandel.py

@@ -0,0 +1,14 @@
+@micropython.native
+def in_set(c):
+    z = 0
+    for i in range(40):
+        z = z*z + c
+        if abs(z) > 60:
+            return False
+    return True
+
+for v in range(31):
+    line = []
+    for u in range(91):
+        line.append('*' if in_set((u / 30 - 2) + (v / 15 - 1) * 1j) else ' ')
+    print(''.join(line))