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/wasi-nn/Cargo.toml

@@ -9,7 +9,7 @@ categories = ["wasm", "computer-vision"]
 keywords = ["webassembly", "wasm", "neural-network"]
 repository = "https://github.com/bytecodealliance/wasmtime"
 readme = "README.md"
-edition = "2018"
+edition = "2021"
 
 [dependencies]
 # These dependencies are necessary for the witx-generation macros to work:

crates/wasi-nn/examples/classification-example/Cargo.toml

@@ -3,7 +3,7 @@ name = "wasi-nn-example"
 version = "0.19.0"
 authors = ["The Wasmtime Project Developers"]
 readme = "README.md"
-edition = "2018"
+edition = "2021"
 publish = false
 
 [dependencies]