std: make sure HashMap from_iter uses random initialization by default

It turns out that HashMap's from_iter implementation was being
initialized without the sip keys being randomized. This adds
a custom default hasher that should avoid this potential vulnerability.
std: micro optimize Hash{Map,Set}::{new,with_capacity}
std: move the hash docstring over to std::hash.
auto merge of #15257 : erickt/rust/hashmap, r=alexcrichton

While `HashMap::new` and `HashMap::with_capacity` were being initialized with a random `SipHasher`, it turns out that `HashMap::from_iter` was just using the default instance of `SipHasher`, which wasn't randomized. This closes that bug, and also inlines some important methods.