Tweak and extend insert sorted by{,_key} methods

[cuviper]

• It's generally better for closures to be the last argument, if only for cleaner formatting
• These methods make sense on VacantEntry as well, since that already has insert_sorted
• insert_sorted_by now takes two entries for comparison, so you don't have to capture a separate copy of the key/value that you're inserting. This is also consistent with the existing sort_by and sorted_by methods.