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

crates/cache/Cargo.toml

@@ -6,7 +6,7 @@ description = "Support for automatic module caching with Wasmtime"
 license = "Apache-2.0 WITH LLVM-exception"
 repository = "https://github.com/bytecodealliance/wasmtime"
 documentation = "https://docs.rs/wasmtime-cache/"
-edition = "2018"
+edition = "2021"
 
 [dependencies]
 anyhow = "1.0"

crates/cache/src/worker.rs

@@ -230,7 +230,6 @@ impl WorkerThread {
 
     #[cfg(target_os = "windows")]
     fn lower_thread_priority() {
-        use std::convert::TryInto;
         use winapi::um::processthreadsapi::{GetCurrentThread, SetThreadPriority};
         use winapi::um::winbase::THREAD_MODE_BACKGROUND_BEGIN;