wasmtime: Initial, partial support for externref

This is enough to get an `externref -> externref` identity function
passing.

However, `externref`s that are dropped by compiled Wasm code are (safely)
leaked. Follow up work will leverage cranelift's stack maps to resolve this
issue.

crates/c-api/src/store.rs

@@ -1,10 +1,10 @@
 use crate::wasm_engine_t;
-use wasmtime::{HostRef, InterruptHandle, Store};
+use wasmtime::{InterruptHandle, Store};
 
 #[repr(C)]
 #[derive(Clone)]
 pub struct wasm_store_t {
-    pub(crate) store: HostRef<Store>,
+    pub(crate) store: Store,
 }
 
 wasmtime_c_api_macros::declare_own!(wasm_store_t);
@@ -13,7 +13,7 @@ wasmtime_c_api_macros::declare_own!(wasm_store_t);
 pub extern "C" fn wasm_store_new(engine: &wasm_engine_t) -> Box<wasm_store_t> {
     let engine = &engine.engine;
     Box::new(wasm_store_t {
-        store: HostRef::new(Store::new(&engine.borrow())),
+        store: Store::new(&engine),
     })
 }
 
@@ -29,7 +29,7 @@ pub extern "C" fn wasmtime_interrupt_handle_new(
     store: &wasm_store_t,
 ) -> Option<Box<wasmtime_interrupt_handle_t>> {
     Some(Box::new(wasmtime_interrupt_handle_t {
-        handle: store.store.borrow().interrupt_handle().ok()?,
+        handle: store.store.interrupt_handle().ok()?,
     }))
 }