no_std support (closes #11)

[tarcieri]

Changes references from std:: to core:: where applicable, ensuring the crate builds in no_std environments.

[tarcieri]

Wow, using std::os::raw::c_void from no_std is one deep rabbit hole. Some decent background in this thread:

https://internals.rust-lang.org/t/solve-std-os-raw-c-void/3268/16

I guess this is the path forward, but work has not yet begun:

rust-lang/rust#43467

An interim option is to use libc::c_void from the libc crate, but no_std users are presently stuck on nightly anyway for a number of reasons, so simply using the nightly features seems to me like the best solution for no_std users for now.

[tarcieri]

On second thought, the std feature I added exists exclusively to support this. It'd be nice to not have a std feature at all.

An interim solution would be to change the no_cc feature to be a default cc feature (features are supposed to be additive anyway), and have an optional dependency on libc pulled in when the cc feature is enabled.

[tarcieri]

Well, I redid this as a breaking change (see f012580 for an attempt to do it mostly backwards compatibly).

I changed the no_cc feature to a cc feature (cargo features should be additive anyway), and used that to gate pulling in the libc, as that seems like the best way to get at c_void for now. It seems this is a bit of a quagmire, and std::os::raw should be deprecated, but there isn't a good replacement now besides the libc crate:

https://internals.rust-lang.org/t/solve-std-os-raw-c-void/3268/16

Apparently this is the path forward:

rust-lang/rust#43467

[cesarb]

You missed the documentation comments and the readme, which also reference no_cc.

While void * is more elegant (and core really should have a way to represent it), the C function clear_on_drop_hide is an implementation detail. It would be simple to change its signature to unsigned char *clear_on_drop_hide(unsigned char *ptr), and on the Rust side use u8 instead of c_void (although char * and c_char would be better than unsigned char and u8, but again we would need c_char). That way, you don't need the libc crate, and can use the C function variant even with core (the fallback is not as reliable, best to avoid it if at all possible).

[tarcieri]

Okay, I changed clear_on_drop_hide's signature to use unsigned char *ptr and changed the Cargo feature back to no_cc (I still think a positive cc feature would make more sense, but that's a topic for a separate PR if it can be avoided in this one)