wiggle: choose between &mut self and &self (#5428)

Previously, all Wiggle-generated traits were generated with `&mut self`
signatures. With the addition of the `mutable` configuration option to
`from_witx!` and `wasmtime_integration!`, one can disable this, emitting
instead traits that use `&self` (i.e., `mutable: false`). This change is
helpful for implementing wasi-threads: WASI implementations with
interior mutability will now be able to communitcate this to their
Wiggle-generated code.

The other side of this change is the `get_cx` closure passed to Wiggle's
generated `add_to_linker` function. When `mutability` is set to `true`
(default), the `get_cx` function takes a `&mut` data structure from the
store and returns a corresponding `&mut` reference, usually to a field
of the passed-in structure. When `mutability: false`, the `get_cx`
closure will still take a `&mut` data structure but now will return a
`&` reference.

crates/wiggle/generate/src/wasmtime.rs

@@ -46,11 +46,16 @@ pub fn link_module(
         format_ident!("add_{}_to_linker", module_ident)
     };
 
+    let u = if settings.mutable {
+        quote!(&mut U)
+    } else {
+        quote!(&U)
+    };
     quote! {
         /// Adds all instance items to the specified `Linker`.
         pub fn #func_name<T, U>(
             linker: &mut wiggle::wasmtime_crate::Linker<T>,
-            get_cx: impl Fn(&mut T) -> &mut U + Send + Sync + Copy + 'static,
+            get_cx: impl Fn(&mut T) -> #u + Send + Sync + Copy + 'static,
         ) -> wiggle::anyhow::Result<()>
             where
                 U: #ctx_bound #send_bound