Update method signatures to better match scoped singletons

[hannobraun]

Now that scoped singletons are a thing, it makes sense to change some method signatures from taking &self to taking &mut self. An example of this is the NVIC API. See comments by @japaric in #62.

I plan to take a look at this, but I won't be able to any time soon. If anyone wants to pick this up, go ahead!