Upgrade hashbrown to 0.15.0

[utay]

https://github.com/rust-lang/hashbrown/blob/master/CHANGELOG.md

Some performance improvements. Also in 0.15.0 there's this allocation_size method that returns the total amount of memory allocated in bytes; I think the SizeOf implementation is still useful because allocation_size is implemented only under certain conditions:

impl<K, V, S, A> HashMap<K, V, S, A>
where
    K: Eq + Hash,
    S: BuildHasher,
    A: Allocator,
{
...