We split off the evaluation engine part of nu-cli into its own crate. This helps improve build times for nu-cli by 17% in my tests. It also helps us see a bit better what's the core engine portion vs the part specific to the interactive CLI piece.
There's more than can be done here, but I think it's a good start in the right direction.
* nu-stream is building on its own, now clean up Cargo.toml
* replace the stream crate in nu-cli
* cc
* since we moved stream out of the nu-cli crate and into its own crate we need to remove pub(crate) and just make it pub
* clean up the prelude and hand merge everything together
* clean up Cargo.tom
* cargo fmt along with Cargo.lock
