Everything in src/common has been moved to src/uucore. This is defined
as a Cargo library, instead of directly included. This gives us
flexibility to make the library an external crate in the future.
Fixes#717.
Builds the uutils multicall binary containing all utils (except stdbuf)
by default. To only build a subset
`cargo --no-default-features --features <utils>`
can be used.
Whats missing is building the standalone binaries and a mechanism to
automatically disable the build of unix only utils on windows.