Filtering and rejecting object keys

Unless you can point me to a simpler way of filtering objects by keys, I'd like to propose the following.

Obj.only
--------

Like `Obj.filter`, but acts on the key.

```ls
only (== \b), {a: 3, b: 4, c: 0} #=> {b: 4}
```

Obj.omit
--------

Like `Obj.reject`, but acts on the key.

```ls
omit (== \b), {a:1, b:2} #=> {a: 1}
```