`alloc`-only `HashMap`?

[RalfJung]

@JarredAllen

My proposal was that code with only alloc would be able to use HashMaps but would have to provide an explicit BuildHasher. This could pull in randomness from some source, or depending on the code it might not need randomness at all if the keys are trusted input data.

This was proposed many times, and IIRC the main blocker is that we have no way of then making the std::collections::hash_map::DefaultHasher the default hasher when using the hash map in std.