Implement RFC 11: Redesigning Wasmtime's APIs (#2897)

Implement Wasmtime's new API as designed by RFC 11. This is quite a large commit which has had lots of discussion externally, so for more information it's best to read the RFC thread and the PR thread.

crates/wiggle/tests/keywords.rs

@@ -33,7 +33,7 @@ mod module_trait_fn_and_arg_test {
     });
     impl<'a> self_::Self_ for WasiCtx<'a> {
         #[allow(unused_variables)]
-        fn fn_(&self, use_: u32, virtual_: u32) {
+        fn fn_(&mut self, use_: u32, virtual_: u32) {
             unimplemented!();
         }
     }