Implement names_vary for pivot_wider()

[DavisVaughan]

Closes #839

Implements names_vary, as described compactly by this comment #839 (comment)

The actual coding of this is fairly straightforward. It just swaps vec_rep() for vec_rep_each() and vice versa while creating the spec columns.

As shown in #839 (comment), this can currently be accomplished using arrange() on a spec generated by build_wider_spec(). But this feels like it might be worth adding an argument for, since it seems to come up fairly often (2 issues and at least 2 stack overflow questions).

[DavisVaughan]

One of the reasons I think this is useful is that, when combined with names_glue, it allows us to generate all 4 of the "common" combinations of names that a user might want

library(tidyr)

# - {values_from}_{names_from}
# - names_from varies fastest
us_rent_income %>% 
  pivot_wider(
    names_from = variable, 
    values_from = c(estimate, moe)
  ) %>%
  names()
#> [1] "GEOID"           "NAME"            "estimate_income" "estimate_rent"  
#> [5] "moe_income"      "moe_rent"

# - {names_from}_{values_from}
# - names_from varies fastest
us_rent_income %>% 
  pivot_wider(
    names_from = variable, 
    values_from = c(estimate, moe),
    names_glue = "{variable}_{.value}"
  ) %>%
  names()
#> [1] "GEOID"           "NAME"            "income_estimate" "rent_estimate"  
#> [5] "income_moe"      "rent_moe"

# - {values_from}_{names_from}
# - names_from varies slowest
us_rent_income %>% 
  pivot_wider(
    names_from = variable, 
    values_from = c(estimate, moe),
    names_vary = "slowest"
  ) %>%
  names()
#> [1] "GEOID"           "NAME"            "estimate_income" "moe_income"     
#> [5] "estimate_rent"   "moe_rent"

# - {names_from}_{values_from}
# - names_from varies slowest
us_rent_income %>% 
  pivot_wider(
    names_from = variable, 
    values_from = c(estimate, moe),
    names_vary = "slowest",
    names_glue = "{variable}_{.value}"
  ) %>%
  names()
#> [1] "GEOID"           "NAME"            "income_estimate" "income_moe"     
#> [5] "rent_estimate"   "rent_moe"

^{Created on 2021-12-14 by the reprex package (v2.0.1)}

[DavisVaughan]

@hadley I'll leave this up to you as to whether you think this is the best way forward for that issue

[hadley]

I like it!