make unwinding an optional feature

[thestinger]

Crate metadata can contain a boolean specifying if the crate supports unwinding and this would be opt-in during compilation via -C unwinding. A crate linking against crates with unwinding enabled should implicitly use unwinding. Since the default is to link against the standard library, nothing will change for most users. The compiler can prevent mixing crates together with a mismatch in the unwinding setting.

[steveklabnik]

Closing in favor of rust-lang/rfcs#471