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/codegen/Cargo.toml

@@ -10,7 +10,7 @@ categories = ["no-std"]
 readme = "README.md"
 keywords = ["compile", "compiler", "jit"]
 build = "build.rs"
-edition = "2018"
+edition = "2021"
 
 [dependencies]
 cranelift-codegen-shared = { path = "./shared", version = "0.83.0" }