py/native: Improve support for bool type in viper functions.

Variables with type bool now act more like an int, and there is proper
casting to/from Python objects.

tests/micropython/viper_types.py

@@ -0,0 +1,26 @@
+# test various type conversions
+
+import micropython
+
+# converting incoming arg to bool
+@micropython.viper
+def f1(x:bool):
+    print(x)
+f1(0)
+f1(1)
+f1([])
+f1([1])
+
+# taking and returning a bool
+@micropython.viper
+def f2(x:bool) -> bool:
+    return x
+print(f2([]))
+print(f2([1]))
+
+# converting to bool within function
+@micropython.viper
+def f3(x) -> bool:
+    return bool(x)
+print(f3([]))
+print(f3(-1))

tests/micropython/viper_types.py.exp

@@ -0,0 +1,8 @@
+False
+True
+False
+True
+False
+True
+False
+True