Until this patch, our C API comes in 2 flavors: `deprecated` and
`wasm_c_api`. With the coming 2.x version of Wasmer, we would like to
remove the `deprecated` API, and keep the `wasm_c_api` only.
This patch removes the `deprecated` API from the `wasmer-c-api`
crate. It also cleans up the `Makefile` and the documentation
system. Previously, the documentation for the `deprecated` API was
relying on Doxygen, which was one new dependency the user had to
install. For the `wasm_c_api`, it relies on `rustdoc`, which is way
better because all examples are run and tested as part of our test
suite.
This clean up also removes the need to deal with `system-libffi` both
in the crate itself and in the `Makefile`, which was an edge case for
macOS on aarch64, and a needle in the foot for some of our users.
Finally, the `build.rs` is now simplified because we no longer need to
exclude symbols from one header to another. It also means that we only
provide the `wasmer_wasm.h` header file now; the `wasmer.h` and
`wasmer.hh` headers are removed.
