py/emitnative: Make viper funcs run with their correct globals context.

Viper functions will now capture the globals at the point they were defined
and use these globals when executing.

tests/micropython/viper_globals.py

@@ -0,0 +1,19 @@
+# test that viper functions capture their globals context
+
+gl = {}
+
+exec("""
+@micropython.viper
+def f():
+    return x
+""", gl)
+
+# x is not yet in the globals, f should not see it
+try:
+    print(gl['f']())
+except NameError:
+    print('NameError')
+
+# x is in globals, f should now see it
+gl['x'] = 123
+print(gl['f']())

tests/micropython/viper_globals.py.exp

@@ -0,0 +1,2 @@
+NameError
+123