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

examples/fib-debug/wasm/Cargo.toml

@@ -2,7 +2,7 @@
 name = "example-fib-debug-wasm"
 version = "0.0.0"
 authors = ["The Wasmtime Project Developers"]
-edition = "2018"
+edition = "2021"
 publish = false
 
 [lib]

examples/tokio/wasm/Cargo.toml

@@ -2,7 +2,7 @@
 name = "example-tokio-wasm"
 version = "0.0.0"
 authors = ["The Wasmtime Project Developers"]
-edition = "2018"
+edition = "2021"
 publish = false
 
 [[bin]]

examples/wasi/wasm/Cargo.toml

@@ -2,7 +2,7 @@
 name = "example-wasi-wasm"
 version = "0.0.0"
 authors = ["The Wasmtime Project Developers"]
-edition = "2018"
+edition = "2021"
 publish = false
 
 [[bin]]