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

cranelift/entity/Cargo.toml

@@ -9,7 +9,7 @@ repository = "https://github.com/bytecodealliance/wasmtime"
 categories = ["no-std"]
 readme = "README.md"
 keywords = ["entity", "set", "map"]
-edition = "2018"
+edition = "2021"
 
 [dependencies]
 serde = { version = "1.0.94", features = ["derive"], optional = true }