Upgrade all crates to the Rust 2021 edition (#3991)

* Upgrade all crates to the Rust 2021 edition

I've personally started using the new format strings for things like
`panic!("some message {foo}")` or similar and have been upgrading crates
on a case-by-case basis, but I think it probably makes more sense to go
ahead and blanket upgrade everything so 2021 features are always
available.

* Fix compile of the C API

* Fix a warning

* Fix another warning

tests/all/custom_signal_handler.rs

@@ -111,7 +111,6 @@ mod tests {
                     move |mut caller: Caller<'_, _>| {
                         let mem = caller.get_export("memory").unwrap().into_memory().unwrap();
                         let memory = mem.data(&caller);
-                        use std::convert::TryInto;
                         i32::from_le_bytes(memory[0..4].try_into().unwrap())
                     }
                 });