Allow bond.spy in production code

[gnecula]

While looking at some of the uses of Bond I realized that sometimes you want to spy some intermediate values in production code. With the new design we have the option to break the function there (ugly), or to create an empty function with a spy_point annotation.

I'd say we allow bond.spy in production code. All we need to do is to make sure it does return immediately if not in test.

[gnecula]

Leaving open for rbond to catch up.

[xkrogen]

Fixed in rbond.