Add by to chop()

[hrryt]

Fixes #1490.

Snags:

1. In the absence of both cols and by, should we 'chop everything' as nest() does? I have thrown an error instead.
2. Should chop() use group vars if data is grouped, as nest() does? Would this require a generic?
3. What should happen if a column is selected by both cols and by? For now, the name is duplicated and repaired.

An example for the latter point:

df <- tibble(x = c(1, 1, 1, 2, 2), y = c(2, 1, 2, 3, 4), z = 1:5)
nest(df, data = everything(), .by = y)
#> # A tibble: 4 × 2
#>       y data            
#>   <dbl> <list>          
#> 1     2 <tibble [2 × 3]>
#> 2     1 <tibble [1 × 3]>
#> 3     3 <tibble [1 × 3]>
#> 4     4 <tibble [1 × 3]>
chop(df, everything(), by = y)
#> New names:
#> • `y` -> `y...1`
#> • `y` -> `y...3`
#> # A tibble: 4 × 4
#>   y...1           x       y...3           z
#>   <dbl> <list<dbl>> <list<dbl>> <list<int>>
#> 1     2         [2]         [2]         [2]
#> 2     1         [1]         [1]         [1]
#> 3     3         [1]         [1]         [1]
#> 4     4         [1]         [1]         [1]