Provide C bindings

After working on fontforge/fontforge#3307, and given how simple the woff2 interface is, it would be nice if a C interface was exposed (e.g. something like [this](https://github.com/fontforge/fontforge/blob/7aea5c0fbe5f4f31b9e3d37ce56fd881e3b96575/fontforge/woff2.cc)), instead of having to do that on our side. It would also make it easier to create bindings to other languages which often have C interop.