Implement traits when SparseSecondaryMap uses custom hasher

[michael-p]

This is an addition to #20, making various trait impls (like Index, Serialize, ...) available even when SparseSecondaryMap is used with a custom hasher (previously they were not implemented in that case meaning something like sec_sparse_map[my_key] = 3 would fail to compile.

[orlp]

Hey, sorry for not getting back to you earlier. This is definitely a wanted patch, but a couple things need changes.

I'd really rather not introduce a (dev) dependency on fxhash when this isn't necessary. Could you write a test case that doesn't use a dependency?

Finally, a very mild inconsistency,

slotmap/src/sparse_secondary.rs

Line 929 in b9fa052

impl<K, V, H> Serialize for SparseSecondaryMap<K, V, H>

uses H instead of S.

[orlp]

Oh well, I just ran the tests and realized serde already pulls in so much stuff, fxhash really doesn't hurt.

[orlp]

And I just realized I'm a moron - the 'mild inconsistency' is a necessity because of a name clash.

[michael-p]

Thanks for merging! I'm really glad to see that you're back working on slotmap, it's a very useful and well-written crate!