Update method signatures to better match scoped singletons

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 https://github.com/japaric/cortex-m/pull/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!