Peripherals as scoped singletons

See this RFC for details: japaric/svd2rust#157

- The first commit adapts this crate to the changes in japaric/cortex-m#65 and japaric/svd2rust#158
- ~~The second commit is an alternative implementation of RFC #47 (there's another implementation in #49. This second commit is not required for RFC157 but let us experiment with safe DMA abstractions.~~ postponed

### TODO

- [x] un-bless peripherals as resources. Peripherals as resources were special cased: if resource listed in e.g. `app.tasks.FOO.resources` didn't appear in `app.resources` then it was assumed to be a peripheral and special code was generated for it. This is no longer required under RFC157.

~~This depends on PR japaric/rtfm-syntax#2~~ postponed