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/isle/fuzz/Cargo.toml

@@ -3,7 +3,7 @@ name = "isle-fuzz"
 version = "0.0.0"
 authors = ["Automatically generated"]
 publish = false
-edition = "2018"
+edition = "2021"
 
 [package.metadata]
 cargo-fuzz = true

cranelift/isle/isle/Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 authors = ["The Cranelift Project Developers"]
 description = "ISLE: Instruction Selection and Lowering Expressions. A domain-specific language for instruction selection in Cranelift."
-edition = "2018"
+edition = "2021"
 license = "Apache-2.0 WITH LLVM-exception"
 name = "cranelift-isle"
 readme = "../README.md"

cranelift/isle/islec/Cargo.toml

@@ -2,7 +2,7 @@
 name = "islec"
 version = "0.1.0"
 authors = ["The Cranelift Project Developers"]
-edition = "2018"
+edition = "2021"
 license = "Apache-2.0 WITH LLVM-exception"
 publish = false