Progress bar to access current/parent environment in map functions.

[jcolt45]

Formatted strings for progress bars in map functions could only access global environment. Updated map_, map2_, and pmap_ to add caller = .purrr_user_env by default to the .progress argument to access current/parent environments.

Fixes #1078

[hadley]

Tested interactively with map(1:10, \(i) Sys.sleep(0.5), .progress = list(format = "{x}"))

[hadley]

@jcolt45 sorry for not having the time to review this interactively 😞, but I'm trying to get a purrr release out soon and didn't want to leave your contribution behind. The main change I made was to pull this out into a helper function, and once I did that I realised we could use it to check that .progress was generally in the correct format. Then I realised we didn't need to pass the environment unless there was a custom format which simplified the implementation a bit more.