Add context manager to enable/disable patching

[rjzamora]

Motivated by @vyasr 's suggestion in #46

Adds a simple context/utility to toggle patching behavior. This should make it easier to quickly test if patching is causing a hypothetical bug in the future.

[pentschev]

I like the idea and it generally looks good to me. However, I don't feel experienced enough with the patching mechanism and would prefer to have @vyasr reviewing.

Thanks @rjzamora for working on this, I can see this being very useful for debugging issues in the future. 🙂

[vyasr]

Instead of rewriting the environment each time in the CM, can we load it once at the module level and store it in a module-scope var that is overwritten temporarily in the CM? Modifying the environment each time feels pretty heavyweight.