Rewamp interrupt handling

[et-tommythorn]

Another fall-out of Pull Request #15: the handling of interrupt embeds too much policy into Dromajo. For co-sim, we need to be able to adapt to the DUTs decision on when to take pending interrupts. The way current way is hacked and the better way would be to simply factor out of the instruction execution all handling of interrupts. Instead we leave it to the client to example pending interrupts and raise them as needed.