…` ..

- remove some variants from `Exception` when targeting ARMv6-M

- future proof the ABI of `__EXCEPTIONS` by using a `union` instead of `Option<fn()>` (the `None`
  variant is not guaranteed to always be represented as the value `0`)

- tweak exception names to match CMSIS and the names used in cortex-m v0.5.0

- add an opt-in "device" feature to opt into a device specific build (interrupts must be supplied by
a different crate, or manually by the user).

- drop the `interrupts!` macro. If you don't enable the "device" feature `cortex-m-rt` will bind all
possible interrupts to the default exception handler.

- document the `main` symbol as an alternative to `entry!`
- document `ResetTrampoline`
- fix: `PendSV` is available on ARMv6-M
- document that `entry!` and `exception!` must be called from accessible modules.
- add a deny lint to `entry!` and `exception!` to prevent them from being invoked from inaccessible
  modules.