rustpkg: Pass command-line arguments to rustc

r? @brson 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