Given that it was previously incorrect, explains logic behind request_status_ and last_request_status_ setting. Also takes the opportunity to ensure that NMI is 'sampled' at the same time as IRQ; whether the next thing should be the NMI routine now occurs one cycle before the end of any instruction. That's an assumption for now. Testing to come.
Following the pattern just established by the 6502, puts all implementation specifics beyond the visibility of a human reading Z80.hpp and in subfolders so as to promote the idea that they shouldn't go out of their way.
