Improve test for Lists vs. environments, and $ vs. [[

[MLopez-Ibanez]

The example here:

R6/vignettes/Performance.Rmd

Lines 712 to 723 in 5078678

	```{r}
	lst <- list(x = 10)
	env <- new.env()
	env$x <- 10
	mb_summary(microbenchmark(
	lst = lst$x,
	env = env$x,
	lst[['x']],
	env[['x']]
	))
	```

is too trivial to show the actual differences. A better example would be

lst <- as.list(sample(letters))
names(lst) <- unlist(lst)
env <- new.env()
for (x in lst) assign(x, x, envir=env)
mb_summary(microbenchmark(
  lst = lst$x,
  env = env$x,
  lst[['x']],
  env[['x']]
))

which gives in my computer:

         name median
1        lst 0.6345
2        env 0.2850
3 lst[["x"]] 0.4085
4 env[["x"]] 0.1960