Reuse nodes based on state matching, not sentential form validity

I think that state matching is the only correct strategy for incremental
node reuse that is compatible with the new error recovery algorithm. It's
also simpler than the sentential-form algorithm. With the compressed parse
tables, state matching shouldn't be too conservative of a test.

spec/runtime/parser_spec.cc

@@ -231,7 +231,7 @@ describe("Parser", [&]() {
               "(identifier) "
               "(math_op (number) (member_access (identifier) (identifier))))))");
 
-          AssertThat(input->strings_read, Equals(vector<string>({ " abc.d);", "" })));
+          AssertThat(input->strings_read, Equals(vector<string>({ " + abc.d)", "" })));
         });
       });