Allow downloading static library for platforms where one is available#161

As the docs mention, building Binaryen from source takes a long time to build, prohibitively so in some projects. On the other hand, Binaryen already provides a static library inside the `lib` directory of its official prebuilt releases. Assuming it contains all the same APIs used by this crate, would it be possible to add an opt-in feature that would download & link those static libraries in build.rs instead of building everything from source at least on supported platforms? Given that both source code and prebuilt releases come from the same Github it should be no more dangerous in terms of trusting 3rd-party code than what's happening implicitly right now anyway.