Function `write_u64` seems to be unused

[matthew-wozniczka]

fast_float/include/fast_float/ascii_number.h

Line 128 in 7c19caf

void write_u64(uint8_t *chars, uint64_t val) {

It's only come to my attention because I'm looking forward to MS complaining about the use of memcpy, and that's the only place in this library where it's not obviously clear that it's not overflowing a buffer (even given the name...). Removing it could avoid future hassles.