Support negative `n` in `slice_head` and `slice_tail`

[eutwt]

Supplying a negative number to slice_head or slice_tail results in an error. Could negative numbers be allowed, so that running df %>% slice_[head/tail](n = x) is equivalent to df %>% group_modify(~ [head/tail](., x)) for both positive and negative numbers?

library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union

#### Remove first three rows from each group
mtcars %>% 
  group_by(cyl) %>% 
  group_modify(~ tail(.x, -3))
#> # A tibble: 23 × 11
#> # Groups:   cyl [3]
#>      cyl   mpg  disp    hp  drat    wt  qsec    vs    am  gear  carb
#>    <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#>  1     4  32.4  78.7    66  4.08  2.2   19.5     1     1     4     1
#>  2     4  30.4  75.7    52  4.93  1.62  18.5     1     1     4     2
#>  3     4  33.9  71.1    65  4.22  1.84  19.9     1     1     4     1
#>  4     4  21.5 120.     97  3.7   2.46  20.0     1     0     3     1
#>  5     4  27.3  79      66  4.08  1.94  18.9     1     1     4     1
#>  6     4  26   120.     91  4.43  2.14  16.7     0     1     5     2
#>  7     4  30.4  95.1   113  3.77  1.51  16.9     1     1     5     2
#>  8     4  21.4 121     109  4.11  2.78  18.6     1     1     4     2
#>  9     6  18.1 225     105  2.76  3.46  20.2     1     0     3     1
#> 10     6  19.2 168.    123  3.92  3.44  18.3     1     0     4     4
#> # … with 13 more rows

mtcars %>% 
  group_by(cyl) %>% 
  slice_tail(n = -3)
#> Error: `n` must be a non-missing positive number.

#### Remove last three rows from each group
mtcars %>% 
  group_by(cyl) %>% 
  group_modify(~ head(.x, -3)) 
#> # A tibble: 23 × 11
#> # Groups:   cyl [3]
#>      cyl   mpg  disp    hp  drat    wt  qsec    vs    am  gear  carb
#>    <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#>  1     4  22.8 108      93  3.85  2.32  18.6     1     1     4     1
#>  2     4  24.4 147.     62  3.69  3.19  20       1     0     4     2
#>  3     4  22.8 141.     95  3.92  3.15  22.9     1     0     4     2
#>  4     4  32.4  78.7    66  4.08  2.2   19.5     1     1     4     1
#>  5     4  30.4  75.7    52  4.93  1.62  18.5     1     1     4     2
#>  6     4  33.9  71.1    65  4.22  1.84  19.9     1     1     4     1
#>  7     4  21.5 120.     97  3.7   2.46  20.0     1     0     3     1
#>  8     4  27.3  79      66  4.08  1.94  18.9     1     1     4     1
#>  9     6  21   160     110  3.9   2.62  16.5     0     1     4     4
#> 10     6  21   160     110  3.9   2.88  17.0     0     1     4     4
#> # … with 13 more rows

mtcars %>% 
  group_by(cyl) %>% 
  slice_head(n = -3)
#> Error: `n` must be a non-missing positive number.

^{Created on 2021-07-28 by the reprex package (v2.0.0)}

[grayskripko]

A temporary workaround for head(n=-1)

> tibble(gr=c(1,1,2,2,2), val=1:5) %>% 
+     group_by(gr) %>% 
+     filter(n() - row_number() + 1 >**1**) %>% 
+     ungroup()
# A tibble: 3 x 2
     gr   val
  <dbl> <int>
1     1     1
2     2     3
3     2     4