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/macro/src/lib.rs

@@ -157,6 +157,7 @@ pub fn from_witx(args: TokenStream) -> TokenStream {
         &doc,
         config.wasmtime,
         &config.tracing,
+        config.mutable,
     )
     .expect("validating codegen settings");
 
@@ -198,6 +199,7 @@ pub fn wasmtime_integration(args: TokenStream) -> TokenStream {
         &doc,
         true,
         &config.c.tracing,
+        config.c.mutable,
     )
     .expect("validating codegen settings");