Make fields of `ExceptionFrame` private

[jonas-schievink]

We might want ExceptionFrame to support accessing FPU registers in the future. Adding these would be a breaking change, since all of its fields are public.

When writing an RTOS, it may be useful to be able to modify the stacked registers, which is generally wildly unsafe, but could be still done correctly. To do this, a &mut ExceptionFrame needs to be passed to the handler. With the current API this is immediately unsound, since the code in the handler can set them without requiring unsafe.

Making the fields private and having safe getters and unsafe setters would allow this sort of API to be provided.