Change lib.field() to take field objects, not names

**Reported by [Eevee](http://github.com/eevee) on 2011/04/28 05:56:47 +0000 · Migrated from [Redmine issue 611](http://bugs.veekun.com/issues/611)**

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Names are sticky and require passing the form object and all kinds of absurdities. Just pass the field object.

```
Redmine metadata:
Updated on: 2011/04/28 05:56:47 +0000
Start date: 2011/04/28
```
