I managed to avoid the multicore trampoline by messing with the signature of the core 1 startup function.
While the first couple arguments to a function with the arm C abi are passed in registers, once they're filled up, the rest of the arguments go on the stack; so, I put some dummy arguments before the real arguments to force them to go onto the stack. That allows it to be used directly, without needing the trampoline to move the arguments from the stack to registers.
I also changed the startup function to be generic over the function type passed, which avoids the mess of dealing with `Core1Main` and fat pointers and all that.
* Add basic multicore fifo example
* Add documentation for multicore
* Send system_clock frequency to core1 over FIFO in example
* Add Stack::new() to HAL. Use Stack::new() in example
