FW-1087: provide wirefilter.h C header file and add ffi-ctests crate

[marmeladema]

This new crate is here to provide a set of C based tests that will
serve two purposes:

• Provide some examples how to use the FFI bindings
• Test that those bindings are actually working as intended

Internally, it relies on ffi-ctests/ctests/tests.c file which contains
tests written in C and that is compiled at cargo configuration time
through the use of a build.rs file. This produces a wirefilter_ffi_ctests.so
shared library that is later used in the ffi-ctests/ctests/src/lib.rs file
to call the different tests functions. All of this is done in order to try
to integrate somehow properly with cargo test.

[RReverser]

I'm seeing this upon checkout:

warning: The package `wirefilter_ffi` provides no linkable target. The compiler might raise an error while compiling `wirefilter_ffi_ctests`. Consider adding 'dylib' or 'rlib' to key `crate-type` in `wirefilter_ffi`'s Cargo.toml. This warning might turn into a hard error in the future.
warning: The package `wirefilter_ffi` provides no linkable target. The compiler might raise an error while compiling `wirefilter_ffi_ctests`. Consider adding 'dylib' or 'rlib' to key `crate-type` in `wirefilter_ffi`'s Cargo.toml. This warning might turn into a hard error in the future.

Not sure which change triggers it, but it doesn't look right.

UPD: I guess Rust just expects that if you add it as a dependency, it has to be an rlib too for proper linking. Try to add that to crate-type array.

[marmeladema]

From what i understand, this is because wirefilter-ffi-ctests crate depends on the wirefilter-ffi and the later do not provide a rust-linkable target. One way of removing this warning is to add dylib to the crate type list as mentioned by the warning.

[RReverser]

One way of removing this warning is to add dylib to the crate type list as mentioned by the warning.

Better to use rlib I guess, so that it gets linked as a normal Rust library.

[marmeladema]

I hope everything is fine now 👍

[RReverser]

Pushed commit to run just C tests (which are platform-specific) on Windows and OS X too to make sure our headers stay compatible.