usize instead of u64

[abimek]

Is there a reason why the length argument for the ProgressBar constructor takes in a u64 and not a usize. If not please give me the go ahead and I'll make a PR fixing this (if y'all want).

[djc]

Why do you want it to take a usize? It seems like the kind of thing that people might want more than 4G items for even on 32-but machines, and it also seems like a quantity that's pretty much unrelated to the word size of the target architecture.