Added support for XC8 2.0 and PIC18F2455

[ajkunen]

Thanks for writing this! Here's a contribution

I updated some of the macros to work with the latest XC8 (v2.0 and later) since it adheres more closely with C99, and it sheds some of the old notation. It also requires pragma pack because it seems to want 16-bit alignment (regardless of what the manual says).

I added support for PIC18F2455, which I'm working with right now. Seems to work so far.