feat(c-api) Remove the deprecated API.

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.

lib/c-api/src/wasm_c_api/instance.rs

@@ -184,15 +184,7 @@ pub unsafe extern "C" fn wasm_instance_exports(
     let mut extern_vec = instance
         .exports
         .iter()
-        .map(|(name, r#extern)| {
-            let function = if let Extern::Function { .. } = r#extern {
-                instance.exports.get_function(&name).ok().cloned()
-            } else {
-                None
-            };
-
-            Box::into_raw(Box::new(r#extern.clone().into()))
-        })
+        .map(|(_name, r#extern)| Box::into_raw(Box::new(r#extern.clone().into())))
         .collect::<Vec<*mut wasm_extern_t>>();
     extern_vec.shrink_to_fit();