export rich version information from cargo::version

export rich version information from cargo::version

To support `cargo --version --verbose`, ala rustc, we need more
information to be injected into cargo when it's built from the Makefile,
and a more explicit data structure to be returned from cargo::version.
We implement fmt::Display for our newly-created structure so clients
don't have to bother with the details of interpreting the structure if
all they want is a string.