import some other helpers from pat's crate

* parse the proc macro argument into a file path
* use a much simpler witx spec by default

test.witx

@@ -0,0 +1,13 @@
+
+
+(typename $errno
+    (enum u32
+        $dont_want_to
+        $physically_unable
+        $picket_line))
+
+(module $foo
+  (@interface func (export "bar")
+    (param $name string)
+    (result $error $errno))
+)