Simplify backtrace handling

[nspin]

Currently, the complicating factor is supporting moving backtrace data around without an allocator. This is a requirement, but could be done more simply by just using ArrayVec and requiring the user to specify an upper bound on the length of the backtrace.

Perhaps a trait could be used to abstract over the consumer of frame addresses. A consumer could send them using postcard, assemble them into a Vec, or assemble them into an ArrayVec.