feat(c-api) No longer expand with cbindgen: drop dependency to Rust nightly.

So. Let's explain a dirty hack. `cbindgen` reads the code and collects
symbols. What symbols do we need? None of the one declared in
`wasm.h`, but for non-standard API, we need to collect all of
them. The problem is that `wasm_named_extern_t` is the only
non-standard type where extra symbols are generated by a macro
(`wasm_declare_boxed_vec!`). If we want those macro-generated symbols
to be collected by `cbindgen`, we need to _expand_ the crate
(i.e. running something like `rustc -- -Zunstable-options
--pretty=expanded`). Expanding code is unstable and available only on
nightly compiler. We _don't want_ to use a nightly compiler only for
that. So how can we help `cbindgen` to _see_ those symbols?

First solution: We write the C code directly in a file, which is then
included in the generated header file with the `cbindgen`
API. Problem, it's super easy to get it outdated, and it makes the
build process more complex.

Second solution: We write those symbols in a custom module, that is
just here for `cbindgen`, never used by our Rust code (otherwise it's
duplicated code), with no particular implementation.

And that's why we have the following `cbindgen_hack` module.

But this module must not be compiled by `rustc`. How to force `rustc`
to ignore a module? With conditional compilation. Because `cbindgen`
does not support conditional compilation, it will always _ignore_ the
`#[cfg]` attribute, and will always read the content of the module.

Sorry.

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

@@ -403,7 +403,7 @@ int main() {
             #[no_mangle]
             pub unsafe extern "C" fn [<wasm_ $name _vec_copy>](
                 out_ptr: &mut [<wasm_ $name _vec_t>],
-                in_ptr: & [<wasm _$name _vec_t>])
+                in_ptr: & [<wasm_ $name _vec_t>])
             {
                 *out_ptr = in_ptr.clone();
             }