* addHL and subHL affect flags, and are smaller
Most importantly, addHL and subHL now affect the flags as you would expect from a 16 bit addition/subtraction. This seems like it'd be preferred behaviour, however I realise any code relying on it not affecting flags would break. One byte saved in addHL, and two bytes saved in subHL. Due to the branching nature of the original code, it's difficult to compare speeds, subHL is either 1 or 6 cycles faster depending on branching, and addHL is between -1 and 3 cycles faster. If the chance of a carry is 50%, addHL is expected to be a cycle faster, but for a chance of carry below 25% (so a < 0x40) this will be up to a cycle slower.
* Update core.asm
* Reworked one use of addHL
By essentially inlining both addHL and cpHLDE, 100 cycles are saved, but due to the registers not needing preserving, a byte is saved too.
* Corrected spelling error in comment
* Reworked second use of addHL
43 cycles saved, and no more addHL in critical loops. No bytes saved or used.
* Fixed tabs and spacing, and made a comment clearer.
* Clearer comments
* Adopted push/pop notation
I'm about to split the global registry in two (labels and consts)
and the previous state of registry selection made things murky.
Now it's much better.
During expression parsing, if a local label was parsed, it would
select the local registry and keep that selection, making
subsequent global labels register in the wrong place.
