Add zero padding to FIX_PRINTF_HEX
Add first pass at fix_div_64 (using long division), no inf/nan guards or negative numbers
Add support for overflow detection, negative inputs to fix_div_64
Allow fix_div_64 to handle the largest negative number (in two's complement)
Clean up fix_div, fix division by infinity, make all tests pass
Add tests for inf/inf
Make fix_div_64 constant-time