Better implementation of LinkedList.firstIndexWhere()

See ceylon/ceylon.language#492.

I tested performance of my `ceylon.formatter` formatting [FormattingWriter.ceylon](https://github.com/lucaswerkmeister/ceylon.formatter/blob/21c0a414be3c48fe01f106488ef2fbd55eb7aa4c/source/ceylon/formatter/FormattingWriter.ceylon) 1000 times; results:
```bash
# with my optim, without LinkedList optim:
$ ceylon compile 2>/dev/null; ceylon run ceylon.formatter
37573972913
# with my optim, with LinkedList optim:
$ ceylon compile 2>/dev/null; ceylon run ceylon.formatter
25694939585
# without my optim, with LinkedList optim:
$ git stash save --quiet; git co origin/master --quiet; git stash pop --quiet; ceylon compile 2>/dev/null; ceylon run ceylon.formatter
27968390451
# without my optim, without LinkedList optim:
$ ceylon compile 2>/dev/null; ceylon run ceylon.formatter
37539447877
```
(“my optim” is a yet unpushed `ceylon.formatter` commit that replaces two uses of `firstIndexWhere` with a hand-written lookup; “LinkedList optim” is this change.)