Support --use-core and --ctypes-prefix

[emilio]

As requested in https://github.com/Yamakaky/rust-bindgen/issues/400#issuecomment-258632805.

r? @fitzgen

[emilio]

poor man's libc :)

[emilio]

@bors-servo delegate=fitzgen

[bors-servo]

✌️ @fitzgen can now approve this pull request

[fitzgen]

LGTM, but one comment below.

[fitzgen]

Shouldn't --use-core imply extern crate core; in the bindings? Shouldn't we automatically insert the extern crate definition?

[emilio]

Nope, at least not completely automatically I guess.

It's not uncommon doing include!("path/to/bindings.rs"), in which case it's unnecessary. In general it's completely unnecessary as long as the bindings aren't the crate root, so I went the easy/safe way. if it's harmless we can do it in a followup, in which case is straight-forward to do.

[alexisvl]

Wow, that was fast. Thank you!

[emilio]

@bors-servo r=fitzgen

[bors-servo]

📌 Commit 6611276 has been approved by fitzgen

[bors-servo]

⌛ Testing commit 6611276 with merge 8c737ec...

[bors-servo]

☀️ Test successful - status-travis

[jdub]

Doesn't seem to output core in every instance, e.g. this is the output of the func_ptr.h test:

/* automatically generated by rust-bindgen */

extern "C" {
    #[link_name = "foo"]
    pub static mut foo:
               ::std::option::Option<unsafe extern "C" fn(x:
                                                              ::std::os::raw::c_int,
                                                          y:
                                                              ::std::os::raw::c_int)
                                         -> ::std::os::raw::c_int>;
}

[emilio]

@jdub good catch, that's a one-off :)

#229