Split into lib and bin

[Manishearth]

This improves compile times since we don't need to effectively create a second
rustc when running cargo build --lib or using clippy as a dependency. cargo build will still be slowly

[Manishearth]

Boiling down the travis issue, it seems like when we cargo install this, it builds a bin depending on a dynamic clippy instead of a static one. Adding crate-type = ["dylib", "rlib"] to the lib section doesn't improve things.

cc @alexcrichton

[alexcrichton]

I believe this may be a limitation of the compiler today. There's a nice fat comment about what we do today, but it boils down to basically a "poor man's heuristic" where if we link anything dynamically (like the compiler) we maximize the number of dynamic dependencies. Which is to say that we prefer the dylib to the rlib when both are found.

You may be able to fix this by having an executable depending on an rlib (the implementation), and the plugin is a dylib also depending on the rlib.