Add `upsert`?!

[sjakobi]

Compare haskell/containers#1145.

The existing insertModifying function seems similar. Maybe upsert could replace it?!

[treeowl]

I believe those functions have equivalent power. I'm not sure how performance compares. And I don't see a compelling need to remove the existing function.

[sjakobi]

insertModifying is an internal function though that AFAIK doesn't have an equivalent in containers.

[treeowl]

Ah... I forgot that, though it's not surprising considering the unboxed tuple in its type.