Nested macro with attribute `internal_macro_calls` requires increasing in case of nested macro calls

[olegnn]

I'm very sorry to worry you again, but the problem with nested macros still exists.
For now, there's no way to invoke nested macro in nested macro without increasing internal_macro_calls. In example below this value in join_all! macro attribute is 5 because in code

#[proc_macro_hack(support_nested, internal_macro_calls = 5)]
pub use proc_macro_hack_bug_impl::join_all;

async fn add_nested_custom_joined_results() -> usize {
    let results = join_all!(ready(join_all!(ready(7usize)).0), ready(join_all!(ready(8usize)).0), ready(join_all!(ready(join_all!(ready(9usize)).0)).0));
    results.0 + results.1 + results.2
}

we have 5 calls of join_all! macro each of which inserts 1 nested join! macro. Of course this problem can be solved by setting internal_macro_calls to maximum value, but...

Unfortunately, in situation with join_all_x2! even value of 60 doesn't help. ===> https://github.com/olegnn/proc_macro_hack_bug

[dtolnay]

I don't know of a way to fix this unfortunately, but I would accept a PR if anyone comes up with something.