Generate Bytes for bytes

[shikhar]

https://docs.rs/prost-build/0.7.0/prost_build/struct.Config.html#method.bytes

This can facilitate fewer copies when working with data-heavy APIs.

[mechiru]

@shikhar Your suggestion is very cool. I'm a fan of the bytes crate.
However, this change is destructive and should be released with caution.
(Although it's not so popular as to release it so carefully 😅)

[shikhar]

Indeed, it would be a breaking change!