save a parser's desugared form when h_desugar is called on it for the first time

src/desugar.c

@@ -2,5 +2,10 @@
 #include "internal.h"
 
 HCFChoice *h_desugar(HAllocator *mm__, const HParser *parser) {
-  return parser->vtable->desugar(mm__, parser->env);
+  if(parser->desugared == NULL) {
+    // we're going to do something naughty and cast away the const to memoize
+    ((HParser *)parser)->desugared = parser->vtable->desugar(mm__, parser->env);
+  }
+
+  return parser->desugared;
 }