Avoid global scope conflicts and rewrites

Background

As discussed in https://gitlab.com/eyeo/adblockplus/adblockpluscore/merge_requests/145, we are currently placing our snippets on the global scope, causing the following potential issues:

• we overwrite our own snippets each time
• we potentially obscure otherwise globally available variables, such as trace, log, or any other snippet function name
• we are incapable of securing within a closure our own code
• we indirectly setup the execution context as strict

What to change

Makes the injection based on a single closure, so that all pain points will be gone in a single shot, and no script can detect our snippets landing on the global scope.