Optimize instance Ord of SourcePos

[ulidtko]

Followup to issue #171.

This is partial remedy, aiding somewhat with "creative" users who pass the entire input into the parse function — both as the input, and as its SourceName / filename. This makes SourceName arbitrarily large — hence, comparing first by the strict-Int fields makes a lot of sense, and does help somewhat (not terribly well) according to measurements.

@parsonsmatt I've credited you in Co-Authored-By — for you've made the observation about the stock-derived Ord instance in the issue thread. Feel free to raise objection if you'd not like that, I'll promptly remove.

[ulidtko]

As a further idea: using something like hashable to guarantee constant-time comparison of SourcePos'es, regardless of how much MiB's of junk data the user has put into the SourceName.

But before implementing that, I'd like to hear from @phadej first. If such an idea sounds any good.

[phadej]

This is breaking change and doesn't fix #171.