Support stable Rust

[hannobraun]

This crate can only be used with a nightly Rust toolchain.

As of 2017-10-12, here's the list of unstable features that we currently depend on:

• global_asm (documentation, tracking issue)
• macro_reexport (documentation, tracking issue)
• used (documentation, tracking issue)
• const_fn (documentation, tracking issue)

I think the thing to do here is just to wait for the features to be stabilized, or for svd2rust to be updated to not use them anymore. Personally, I don't feel in any hurry here.

[hannobraun]

As of #47, this crate uses no experimental features and builds on the beta channel. It should build on stable when the next stable version is released.