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|



Yeah, but when `valid?` is called a few times for every file it adds up. Anyway, the percentage of files with a bogus URI should be really small, so this doesn't matter.

Optimizing how `parts` is called would mean changing lots of code - this PR (and another I'm just uploading) is just a small change that brought the performance down to acceptable levels for me. Feel free to make further optimizations :smile: