Cache the Uri parser instance in Checkable, results in a 14x performance increase

The parts getter in Checkable is called **very** often, which means caching the Uri parser instance instead of recreating it on every call massively improves performance.

Here's a small timing of a site with ~30 files (external checks ignored):

|before|after|
|------------|------------------|
|**real 0m25.782s**|**real 0m1.810s**|
|user 0m25.377s|user 0m1.681s|
|sys 0m0.384s|sys 0m0.122s|



Hmm, I don't think so, but I'm no Ruby expert so please confirm what happens in this scenario:
1.  First call to `parts`, which means `parts_cached` is not set
2. Some error happens during parsing -> rescue -> `parts_cached` is nil
3. Another call to `parts` <-- does it now try to parse the Uri again (and fail) or does it return nil as it should?