py: Fix compiler to handle lambdas used as default arguments.

Addresses issue #1709.

tests/basics/fun_defargs.py

@@ -1,3 +1,5 @@
+# testing default args to a function
+
 def fun1(val=5):
     print(val)
 
@@ -18,3 +20,10 @@ try:
     fun2(1, 2, 3, 4)
 except TypeError:
     print("TypeError")
+
+# lambda as default arg (exposes nested behaviour in compiler)
+def f(x=lambda:1):
+    return x()
+print(f())
+print(f(f))
+print(f(lambda:2))