While it is simpler to pass around `ol.geom.Geometry` instances here, when we want to allow users to create spatial filters, we need to do WKT parsing of strings.

A user would create an expression that could be used as a spatial filter like this:
```js
var expr = ol.expr.parse('intersects("POINT(-110 45)")')
```

This is similar to the following CQL:
```
"INTERSECTS(geometry, POINT (-110 45))"
```

So for now I think it is ok to have these spatial functions accept `ol.geom.Geometry` instances as their first arg.  But since users will never be able to create geometries this way (only `number`, `string`, `boolean`, and `null` are supported), we should add support for a `string` as the first arg.  This would let someone use `ol.expr.parse` when creating a `ol.style.Style` and have it serialized as SLD.