std::hashmap: Tighter representation for the Bucket

[bluss]

Replace Option<Bucket<K, V>> with a custom
enum Bucket { Unused, Filled(hash, key, value) }. The non-nullable
pointer optimization for enums with two variants will be active with the
new representation, while it was not with the old. This is if either
K or V is a non-nullable pointer type.

With these changes, now Bucket<int,int> is 32 bytes (like before) but
Bucket<~str,int> is 24 bytes on 64-bit, so we save the 8 bytes of enum
tag per bucket.

[thestinger]

I think we should just make the the adt.rs code smart enough to do this itself.

[thestinger]

cc @jld

[jld]

We probably should improve the optimization. (And that should get its own issue if it doesn't have one yet.) The monomorphizer would need to know enough about whatever adt.rs is extracting from the type to avoid merging Option<(int, ~int)> and Option<(~int, int)>, or Option<(int[2], ~int)> and Option<(int[3], ~int)>, or whatever.

[thestinger]

@jld: I'm planning on replacing our type_use stuff with LLVM's mergefunc pass by the way, so stuff like this should get easier. #9216 is one of the current blockers.

[bluss]

Closing this Pull unmerged, in favor of the linked issue I opened.