Add cargo.vim compiler file.

This PR adds a new Vim compiler file specifically for use with Cargo. It passes all arguments through, so commands like `:make build`, `:make clean`, and `:make run` all work as expected.

It also adds a quickfix autocommand for fixing the paths before populating the error list. `cargo build` reports errors with file paths that are relative to Cargo.toml, so if you're further down in the project tree, then trying to open the error will result in a blank buffer because Vim treats that path as relative to the working directory instead. With this fix, the paths work properly no matter where you are in the project.