Implement PartialEq and Eq

[overvenus]

Hello again!

This PR implements traits PartialEq and Eq. 😀

Cc #19

[bluss]

I'd like to do this more generally than libstd.

We sometimes have room to fix their mistakes or oversights, right? And we should do it if it remains a drop in replacement.

What I first thought of here is that PartialEq should not require that both maps have the same hasher. But in fact, we can generalize it so that neither K, V, S need to be the same as long as they implement V1: PartialEq<V2> and so on. Does that make sense to you?

For K it's a bit trickier, we need to be able to look up using one key in the other map. I'm not sure what works there without trying it out in the code.

[overvenus]

That is a good idea!

For K, my first thought is std::convert traits and Borrow, but later on I found none of them are suitable here. PartialEq requires symmetric, a == b implies b == a. I'm not sure what is best for K, maybe they should be identical. 🤔

[bluss]

Hm we can wait a bit with K and merge the Equivalent PR. It's a generalization of the Borrow mechanism #10

[bluss]

I'll have to experiment with the K issue

[bluss]

Hm we can merge this since the Equivalent trait is waiting on the next version anyway, so we can do a small type inference break then.

[bluss]

PartialEq should be exercised in quickcheck tests. Ideally you'd just add a new one next to the ones that test equivalence with HashMap. This test can be added by you or I, depending on who gets there first.

Edit: I got there first