Added benchmark skeleton, and at least stubs for all of the necessary functions

src/backends/packrat.c

@@ -0,0 +1,15 @@
+#include "../internal.h"
+
+int h_packrat_compile(HAllocator* mm__, HParser* parser, const void* params) {
+  return 0; // No compilation necessary, and everything should work
+	    // out of the box.
+}
+
+HParseResult *h_packrat_parse(HAllocator* mm__, HParser* parser, HParseState* parse_state) {
+  return NULL; // TODO: fill this in.
+}
+
+HParserBackendVTable h__packrat_backend_vtable = {
+  .compile = h_packrat_compile,
+  .parse = h_packrat_parse
+};