once proc-macro

[matklad]

We should add proc-macro on top of once-cell with the following API

#[once_cell::once]
fn global_map() -> &'static HashMap<u32, &'static str> {
        let mut m = HashMap::new();
        m.insert(0, "foo");
        m.insert(1, "bar");
        m
}

This imo is more transparent than lazy_static's API. Note that, on the call site, there will be a function call instead of Deref, which is less ergonomic, but more obvious: you see where costly initialization might take place.

[matklad]

This now is more-or-less achieavable without macros with Lazy