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/memory.rs

@@ -27,7 +27,7 @@ impl wasm_memory_t {
     }
 
     fn externref(&self) -> wasmtime::ExternRef {
-        self.memory().externref()
+        self.memory().clone().into()
     }
 }
 
@@ -36,7 +36,7 @@ pub extern "C" fn wasm_memory_new(
     store: &wasm_store_t,
     mt: &wasm_memorytype_t,
 ) -> Box<wasm_memory_t> {
-    let memory = HostRef::new(Memory::new(&store.store.borrow(), mt.ty().ty.clone()));
+    let memory = HostRef::new(&store.store, Memory::new(&store.store, mt.ty().ty.clone()));
     Box::new(wasm_memory_t {
         ext: wasm_extern_t {
             which: ExternHost::Memory(memory),