Fix bugs in carry, half carry and overflow flags handling

[ghost]

This is a follow up for PR #2 where I fix a bug in overflow flag handling. This time, I've fixed bugs in the handling of the half carry flag, and even the carry flag for one instruction.

Here is a summary of the new fixes:

• half carry flag was not handled at all in the CCF instruction.
• half carry and carry flags were badly handled in NEG instruction. E.g., NEGating zero would set CF and HF.
• half carry flag was not always correctly computed in ADC and SBC instructions. The carry/borrow was not taken into account int the evaluation of the half carry in arith8().

The emulator now pass all the ZEXDOC tests (the version in z80pack, without the undocumented instructions) with the exception of DAA, but, I think that is because the undocumented behaviours are not emulated.

[jhallen]

Nice! I'm taking your fixes. I had not tried any Z80 conformance tests. I wonder if it's worth emulating undocumented instructions? My impression is that they are not as widely used as say the 6502 ones were.

[ghost]

Ah, the undocumented instructions. We hear a lot about them but we do not see them often.

It could be worth to optimise the emulator. The test suite runs in ~3 minutes with cpm but only 1 with z80pack. I also feel the difference when assembling a 1500 lines program with M80, which take 2.5s against <1s.