Tx: make abort() function public.

[projectgus]

Allows access to abort function after splitting the driver. The equivalent function in FdCan<I, M> is already public, so I think this is safe.

Thanks for all your work developing fdcan, it's much appreciated!

[projectgus]

I'm going out on a limb here, but alternative way to look at this, maybe, is that Mailbox is only part of two public APIs:

• The abort() function on the main struct
• The PTX callback in transmit_preserve()

... and it's not really needed for anything else. So an alternative breaking change could be:

• Convert Mailbox to an internal enum
• Replace abort() with a public function that aborts all transmits (as I don't see how the crate user picks which Mailbox to abort, my code aborts all of them anyway).
• Change the PTX signature to not pass the Mailbox (as, again, needed anywhere else in the existing API).

This would be a breaking change though, and quite opinionated change for a newcomer like me to suggest. I'd be up for contributing this though, FWIW. 😁