Add Vector::insert_ord_by method

[vlmutolo]

Vector::insert_ord_by is a convenience function I find myself frequently needing. It's useful for when you want to insert an element into a Vector using a custom comparison function. It also gives a nice symmetry to have all of binary_search_by, insert_ord_by, and sort_by defined. Right now we're missing insert_ord_by, which this PR implements.

[jneem]

Hi @vlmutolo, are you interested in opening a PR on imbl? I'd be happy to include this; my only comment is that you can use Ordering::reverse to simplify the example.

[vlmutolo]

Sure! Thanks for trying to take on the maintenance. I'll submit a PR tonight.