add_carry took a fixed 8 cycles with a lda/adc/sta pattern
we can instead use bcc to handle the carry-not-set case in just
2 cycles, skipping over the inc which takes 5 cycles on the
carry-is-set case.
Result is 2-7 cycles instead of 8, saving 1-6 cycles twice
per 16-bit multiplication or square.
Neat!
* switched zero-page from hardcoded assignments to symbols
* moved most non-hotpath stuff out to .data
* merged ptr and pixel_ptr
Slight slowdown in Atari800MacX from 5m13s to 5m15s
This is a first step toward moving the UI to C and
adding file and network I/O in C. The fractal core
will remain in assembler as well as the multiplier.
