stat_count() respects uniqueness of x

[teunbrand]

This PR aims to fix #4609.

Briefly, as.factor(x) and vec_unique(x) have different definitions of what constitutes a unique value, because the as.factor(x) casts x as a character, losing any small digits in doubles. This PR uses rowsum() instead of tapply() to count, which preserves the uniqueness as it doesn't cast the group to a factor (also rowsum() is a bit faster).

Reprex from #4609:

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2
df <- data.frame(x = rep(c(1, 2), 5) + rep(c(0, -2.220446e-16), c(4, 1)))

ggplot(df, aes(x)) + geom_bar()
#> Warning: `position_stack()` requires non-overlapping x intervals

^{Created on 2023-11-13 with reprex v2.0.2}

Of course, position_stack() not unreasonably complains about overlapping intervals, but this is a better hint at what might be going on that the current error.

[thomasp85]

LGTM

[thomasp85]

Please add NEWS bullet