Function name collisions for Aragon Apps

[bingen]

No Aragon App will be able to use any of the following function/variable names, because they fall under the AppProxy* contracts in the inheritance graph, and therefore the fallback with the delegated call is not reached if one of them is called:

• kernel
• appId
• setKernel
• setAppId
• implementation
• proxyType
• setPinnedCode
• pinnedCode
• getAppBase
• delegatedFwd
• isContract

It’s probably not a big deal, as it doesn’t seem so likely, some of them belong to standards, and anyway the workaround is so easy as changing the name, but we should document it.
It would have been nice to prefix them all, except the standard ones, with a prefix like aOs, but I don’t think the breaking change is now worth it.

For reference, it was motivated by this comment: #597 (comment)