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/linker.rs

@@ -12,7 +12,7 @@ pub struct wasmtime_linker_t {
 #[no_mangle]
 pub extern "C" fn wasmtime_linker_new(store: &wasm_store_t) -> Box<wasmtime_linker_t> {
     Box::new(wasmtime_linker_t {
-        linker: Linker::new(&store.store.borrow()),
+        linker: Linker::new(&store.store),
     })
 }
 
@@ -87,7 +87,7 @@ pub unsafe extern "C" fn wasmtime_linker_instantiate(
     trap_ptr: &mut *mut wasm_trap_t,
 ) -> Option<Box<wasmtime_error_t>> {
     let result = linker.linker.instantiate(&module.module.borrow());
-    super::instance::handle_instantiate(result, instance_ptr, trap_ptr)
+    super::instance::handle_instantiate(linker.linker.store(), result, instance_ptr, trap_ptr)
 }
 
 #[no_mangle]
@@ -116,6 +116,6 @@ pub unsafe extern "C" fn wasmtime_linker_get_default(
         Err(_) => return bad_utf8(),
     };
     handle_result(linker.get_default(name), |f| {
-        *func = Box::into_raw(Box::new(HostRef::new(f).into()))
+        *func = Box::into_raw(Box::new(HostRef::new(linker.store(), f).into()))
     })
 }