executor: "send-spawn is OK if the args are Send" only holds for async fn futures.

The normal `spawn()` methods can be called directly by the user, with arbitrary hand-implemented futures.
We can't enforce they're only called with `async fn` futures. Therefore, make these
require `F: Send`, and add a "private" one only for use in the macro, which can enforce it.

embassy-macros/src/macros/task.rs

@@ -76,7 +76,7 @@ pub fn run(args: syn::AttributeArgs, f: syn::ItemFn) -> Result<TokenStream, Toke
         #visibility fn #task_ident(#fargs) -> #embassy_path::executor::SpawnToken<impl Sized> {
             type Fut = impl ::core::future::Future + 'static;
             static POOL: #embassy_path::executor::raw::TaskPool<Fut, #pool_size> = #embassy_path::executor::raw::TaskPool::new();
-            POOL.spawn(move || #task_inner_ident(#(#arg_names,)*))
+            unsafe { POOL._spawn_async_fn(move || #task_inner_ident(#(#arg_names,)*)) }
         }
     };