ensure reproducible elements order by sorting

[hboutemy]

extracted elements order non-controversial commit from #209

fixes issues like:

├── org/antlr/grammar/v3/ANTLRv3Parser.java
│ @@ -427,15 +427,15 @@
│  				cnt7++;
│  			}
│
│  			EOF12=(Token)match(input,EOF,FOLLOW_EOF_in_grammarDef489); if (state.failed) return retval;
│  			if ( state.backtracking==0 ) stream_EOF.add(EOF12);
│
│  			// AST REWRITE
│ -			// elements: DOC_COMMENT, attrScope, optionsSpec, rule, tokensSpec, action, id
│ +			// elements: action, tokensSpec, attrScope, rule, optionsSpec, DOC_COMMENT, id
│  			// token labels:
│  			// rule labels: retval
│  			// token list labels:
│  			// rule list labels:
│  			// wildcard labels:
│  			if ( state.backtracking==0 ) {
│  			retval.tree = root_0;