Stop relying on `Hash{Set,Map}::from_iter` directly

Stop relying on `Hash{Set,Map}::from_iter` directly

In the next commit we'll bump the `hashbrown` version, which no
longer randomizes its hasher by default. Thus, we'll need to call
a different constructor in no-std builds from std builds.

Here we do a quick prefactor to use wrappers for `FromIterator`
constructors instead of calling the tables directly to make the
version bump changeset smaller.