hal-core trait for core-local storage

[hawkw]

this would allow other crates, like mycelium-alloc and mycelium-trace, to use core-local storage by being generic over an architecture-specific core-local implementation.

[hawkw]

this unlocks stuff like:

• tracking the current span per core in mycelium-trace
• mimalloc-style sharded heap in mycelium-alloc
• other Hidden Fun Stuff probably