Allow spread_values functions to work with unquoted paths

[jeremystan]

The following works:

'{"key": "value"}' %>% spread_values(key = jstring("key"))
#>   document.id   key
#> 1           1 value

but this does not:

'{"key": "value"}' %>% spread_values(key = jstring(key))
#> Error in as_function(.f, ...) : object 'key' not found

[jeremystan]

This should work with enter_object as well:

# works
'{"k1": {"k2": "value"}}' %>% enter_object("k1")
# should also work
'{"k1": {"k2": "value"}}' %>% enter_object(k1)

[jeremystan]

Looks like we can use the final NSE Dot-dot-dot section here to do this:
https://cran.r-project.org/web/packages/lazyeval/vignettes/lazyeval.html

[jeremystan]

Hadley's suggestion of

library(purrr)

# Create a JSON path with a minimum of typing
path <- function(...) {
  dots <- dots(...)

  if (!every(dots, is.name)) {
    stop("Path components must be single names", call. = FALSE)
  }

  structure(
    map_chr(dots, as.character),
    class = "path"
  )
}

dots <- function(...) {
  eval(substitute(alist(...)))
}

path(a, b, c)

is working great with minor modifications!

[jeremystan]

Need to:

• Use in all other functions
• Change documentation to be predominantly unquoted
• Change vignettes to be unquoted
• Update NEWS and README
• Improve documentation of path itself (and question why it is exported)
• Thank Hadley!