Ship a 1.0 release

Part of https://github.com/rust-embedded/wg/issues/383

Might also involve https://github.com/rust-lang/rust/issues/27731 so we can get rid of (some) of the inline asm / pre-compiled objects.

• Consider getting rid of aligned dependency subtree (https://github.com/rust-embedded/cortex-m/issues/184)
• Remove the const-fn feature
• Remove all currently #[deprecated] APIs
• Land #181
  • Audit the crate for other things that should be #[cfg]d out depending on the target
• Make all types #[non_exhaustive] where it makes sense
• Use an svd2rust PAC instead of doing register access by hand
• Improve documentation of the Peripherals::steal() API, or redesign that API (https://github.com/rust-embedded/cortex-m/issues/186)
• Make usage from multi-core chips sound
• Document that the inline-asm feature is completely unstable and semver-exempt
• Document availability of target-specific APIs (using #[doc(cfg(X))] where possible)
• Fix soundness bug in cache maintenance API https://github.com/rust-embedded/cortex-m/issues/188
• Fix soundness bugs around MMIO in general (https://github.com/rust-embedded/wg/pull/387)
• Review against API guidelines
• Update to bare-metal 1.0 (one released)
• ...

Start with the remaining checklist in issue #183 and review the linked issues for multi-core support, Peripherals::steal(), MMIO soundness, and the working-group release context. Compare the crate against the Rust API Guidelines and the listed bare-metal update requirements. Done means the unresolved release checklist items are addressed and the crate is ready for a 1.0 release.