rustpkg now accepts most of rustc's command-line arguments and passes
them along to rustc when building or installing.
A few rarely-used arguments aren't implemented yet.
rustpkg doesn't support flags that don't make sense with rustpkg
(for example, --bin and --lib, which get inferred from crate file names).
Closes#8522
`rustpkg build`, if executed in a package source directory inside
a workspace, will now build that package. By "inside a workspace"
I mean that the parent directory has to be called `src`, and rustpkg
will create a `build` directory in .. if there isn't already one.
Same goes for `rustpkg install` and `rustpkg clean`.
For the time being, `rustpkg build` (etc.) will still error out if
you run it inside a directory whose parent isn't called `src`.
I'm not sure whether or not it's desirable to have it do something
in a non-workspace directory.
