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.

build.rs

@@ -205,6 +205,7 @@ fn ignore(testsuite: &str, testname: &str, strategy: &str) -> bool {
 
             // Still working on implementing these. See #929.
             ("reference_types", "table_copy_on_imported_tables") => return false,
+            ("reference_types", "externref_id_function") => return false,
             ("reference_types", _) => return true,
 
             _ => {}