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.

tests/misc_testsuite/reference-types/externref-id-function.wast

@@ -0,0 +1,8 @@
+(module
+  (func (export "identity") (param externref) (result externref)
+    local.get 0))
+
+(assert_return (invoke "identity" (ref.null extern))
+               (ref.null extern))
+(assert_return (invoke "identity" (ref.extern 1))
+               (ref.extern 1))