Let-bind mailbox_code for debugging.

Arranging for this value to have its own variable name in the generated
C code means I can find it more easily in the generated source and I can
inspect its value in the debugger.
Bugfix: CAN data-length code is Ix 9, not Ix 8.

Because values of type (Ix n) range over [0, n-1].
bxCAN: Leave sleep mode at start of initialization.

I can't tell whether this matters, but it seems reasonable and doesn't
seem to make my problems any worse.
bxCAN: Don't wait for the peripheral to finish init.

Once we've programmed the registers we can start enqueuing messages to
transmit immediately. They won't actually go out until the peripheral
finishes initializing, but they'll go out eventually.