rewrite fixations_to_timepoints (fka binifyFixations) using join_by

Note: `fixations_to_timepoints` isn't yet implemented at all.

```r
t_series <- fixations %>%
summarise(t_min = min(current_fix_start),
t_max = max(current_fix_end)) %>%
mutate(across(c(t_min, t_max),
~ floor(. / bin_size) * bin_size)) %>%
mutate(t = list(seq(t_min, t_max, by = bin_size))) %>%
select(t) %>%
unnest(cols = t)

t_series %>%
inner_join(
fixations %>%
mutate(across(c(current_fix_start, current_fix_end),
~ floor(. / bin_size) * bin_size)),
by = join_by(between(t, current_fix_start, current_fix_end))
)
```

# Update June 7 2024

The main part (speeding up by switching to a join) was done in a632aa0f.
- [ ] Check the stuff from the [comment below](https://github.com/BergelsonLab/blabr/issues/44#issuecomment-2099209749)
- [ ] see a632aa0f's message for how the function can be further sped up.