Simon Owen

I've made some test changes to give the behaviour described above in a [halt_tweak branch](https://github.com/kosarev/z80/tree/halt_tweak). It may be incomplete but it was enough to pass the test I wrote to...

Rather than asserting, could the default implementation behave as though FF is read from the bus to give RST &38? From what I remember from testing IM 0 and IM...