ProgressBar::WithProgress use #size when available

[notEthan]

Since #count iterates the enumerable, it can be quite slow. #size is not always available but when it is, it can be much faster than #count.
For some large enumerables I work with, iterating the whole thing takes minutes, so doing it once for count and once for the actual iteration is terribly slow. These use an Enumerator with a block specifying size, and that is computed in relatively insignificant time, so using than when it is available is much better.

[paul]

I think this used count over size originally because when it was written, ActiveRecord::Relation#size would load the entire query, but #count would execute the COUNT SQL query. It doesn't look like that's the case any more: ActiveRecord::Relation#size

Is there any reason to use #count at all? Should it just be switched it #size in every case?

[notEthan]

#size is not always defined - the Enumerable module defines #count but not #size; most classes that include Enumerable do define #size, but not all. And the Enumerator class does define #size but it returns nil unless the Enumerator is initialized with a block indicating the size.

[notEthan]

Think this could make it in? I'm working with large enumerables again and having this in a release would be convenient.