Simplify/speed up retrieving of properties

Simplify/speed up retrieving of properties

The hasProperty()/get() sequence used so far is as in the
spec, but requires us to lookup the name twice. Instead add
a bool hasProperty() to Object::__get__() and use that.

Speeds up fact.2.js by ~20%

Change-Id: Ic8c84718f1a702c3da9487010c0d6dd0fee44609
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>