Boxable async fn

Impact

• Able to easily cause some async functions, blocks, or closures to allocate their stack space lazilly when called (by 'boxing' it)
  • Combined with profiler or other tooling support, this can help to tune the size of futures
• Boxed async blocks allows particular portions of a function to be boxed, e.g. cold paths

Design notes

Example might be to use a decorator:

#![allow(unused)]
fn main() {
#[boxed]
async fn foo() { }
}

This does not have to desugar to -> Box<dyn Future<...>>; it can instead desugar to Box<impl Future>, or perhaps a nominal type to permit recursion.