Stream cargo output instead of buffering it all at once

[Kobzol]

@pkolaczk Can you please check that this solves your issue? You can install this version with this command:

$ cargo install --git https://github.com/kobzol/cargo-pgo --branch cargo-stream

The output from criterion is not colored, I'm not sure how to do it yet. Setting cargo --color always doesn't help.

Fixes: #19