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/module_trait.rs

@@ -81,10 +81,15 @@ pub fn define_module_trait(m: &Module, settings: &CodegenSettings) -> TokenStrea
             quote!(async)
         };
 
-        if is_anonymous {
-            quote!(#asyncness fn #funcname(&mut self, #(#args),*) -> #result; )
+        let self_ = if settings.mutable {
+            quote!(&mut self)
         } else {
-            quote!(#asyncness fn #funcname<#lifetime>(&mut self, #(#args),*) -> #result;)
+            quote!(&self)
+        };
+        if is_anonymous {
+            quote!(#asyncness fn #funcname(#self_, #(#args),*) -> #result; )
+        } else {
+            quote!(#asyncness fn #funcname<#lifetime>(#self_, #(#args),*) -> #result;)
         }
     });