Reexport wasmtime::hash_{map,set} from hashbrown internally

[fitzgen]

Some modules were using std::collections::{HashMap, HashSet} when they knew that they were only compiled when the std feature was enabled. But we need to use hashbrown in certain other places (Cranelift's e-graphs, pooling allocator's index allocator, etc.), so we should simply use hashbrown everywhere. This should reduce binary size some small amount, but also means that we never have to ask ourselves "which hash map should I import?" when adding a new hash map import: always use crate::hash_map::HashMap et al.

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