Add output describing different build stages, switch to stderr

[austinabell]

Noticed with #36 we can improve the build process by adding logs indicating which part of the build process is happening, so it's a little less opaque.

Indicating the ABI generation step, build step, and anything else would be useful (also for debugging)

Also, noticed we have print statements, and to match the Rust output (which is to stderr), these added statements and the ones that already exist should use eprintln! rather than println!.

Not a big deal, but if someone is trying to redirect the output of the build step, it might be very awkward to work with stdout and stderr