feat(c-api) Implement wasm_extern_copy.

This patch also moves `wasm_extern_delete` into the correct Rust module.

lib/c-api/src/wasm_c_api/wasi/mod.rs

@@ -524,12 +524,6 @@ pub unsafe extern "C" fn wasi_get_start_function(
     }))
 }
 
-/// Delete a `wasm_extern_t` allocated by the API.
-///
-/// cbindgen:ignore
-#[no_mangle]
-pub unsafe extern "C" fn wasm_extern_delete(_item: Option<Box<wasm_extern_t>>) {}
-
 #[cfg(test)]
 mod tests {
     use inline_c::assert_c;