Replace progress_bar with custom utility function

[JustAman62]

Removes the progress_bar dependency and replaces its single usage with a small utility function which renders a somewhat similar progress bar when downloading models.

Example output snippet (terminal was sized to 50cols to make this easy to read):

iex(2)> Bumblebee.load_model({:hf, "google-bert/be
rt-base-uncased"})
|                                    |  1% 4/440MB
|                                    |  2% 8/440MB
|=                                  |  3% 13/440MB
|=                                  |  4% 17/440MB
|=                                  |  5% 22/440MB
|==                                 |  6% 26/440MB
|==                                 |  7% 30/440MB
|==                                 |  8% 35/440MB
|===                                |  9% 39/440MB
|===                                | 10% 44/440MB

I've tried to implement just enough for this download use case and not much more, except for making sure the method doesn't failover on weird inputs, and supports non-byte unit progress.

This is my first contribution to this lib so please do let me know if I've forgotten to do something, or am not matching the code style in any way :)

Relates to #458.