Add less-allocating args variants

[mahkoh]

fn argv() -> CVec<*mut u8>

fn argv() -> Vec<&'static u8>

[steveklabnik]

I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#862