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/c-api/Cargo.toml

@@ -6,7 +6,7 @@ description = "C API to expose the Wasmtime runtime"
 license = "Apache-2.0 WITH LLVM-exception"
 repository = "https://github.com/bytecodealliance/wasmtime"
 readme = "README.md"
-edition = "2018"
+edition = "2021"
 publish = false
 
 [lib]

crates/c-api/macros/Cargo.toml

@@ -2,7 +2,7 @@
 name = "wasmtime-c-api-macros"
 version = "0.19.0"
 authors = ["The Wasmtime Project Developers"]
-edition = "2018"
+edition = "2021"
 publish = false
 
 [lib]

crates/c-api/src/func.rs

@@ -104,6 +104,7 @@ pub unsafe extern "C" fn wasm_func_new_with_env(
 ) -> Box<wasm_func_t> {
     let finalizer = crate::ForeignData { data, finalizer };
     create_function(store, ty, move |params, results| {
+        drop(&finalizer); // move entire finalizer into this closure
         callback(finalizer.data, params, results)
     })
 }
@@ -233,6 +234,8 @@ pub(crate) unsafe fn c_callback_to_rust_fn(
 ) -> impl Fn(Caller<'_, crate::StoreData>, &[Val], &mut [Val]) -> Result<(), Trap> {
     let foreign = crate::ForeignData { data, finalizer };
     move |mut caller, params, results| {
+        drop(&foreign); // move entire foreign into this closure
+
         // Convert `params/results` to `wasmtime_val_t`. Use the previous
         // storage in `hostcall_val_storage` to help avoid allocations all the
         // time.
@@ -295,6 +298,7 @@ pub(crate) unsafe fn c_unchecked_callback_to_rust_fn(
 ) -> impl Fn(Caller<'_, crate::StoreData>, *mut ValRaw) -> Result<(), Trap> {
     let foreign = crate::ForeignData { data, finalizer };
     move |caller, values| {
+        drop(&foreign); // move entire foreign into this closure
         let mut caller = wasmtime_caller_t { caller };
         match callback(foreign.data, &mut caller, values) {
             None => Ok(()),