message: fix(c-api) `wasm_val_t` is not an opaque type, let's use a C representation.
diff: @@ -20,9 +20,10 @@ pub union wasm_val_inner {
  /// cbindgen:ignore
  #[allow(non_camel_case_types)]
  +#[repr(C)]
  pub struct wasm_val_t {
  - pub(crate) kind: wasm_valkind_t,
  - pub(crate) of: wasm_val_inner,
  + pub kind: wasm_valkind_t,
  + pub of: wasm_val_inner,
  }
  impl Clone for wasm_val_t {
  