Add `FromIterator` and `Extend` impls

[Jules-Bertholet]

These match the impls on HashMap and BTreeMap in the standard library.

[sdd]

Thanks for the PR Jules, much appreciated 👍🏼

I'd thought about implementing FromIterator before but I was unaware of iter.size_hint() :-)

I'd like some unit tests in here first before merging. I can add them when I get time to do so, or feel free to add a couple of basic ones if you want it merged faster :-)