Refactor .get().map(Root::from_rooted)
and .get().map(|foo| foo.root())
to .get_rooted() on MutNullableHeap objects.

First part done mechanically with the following comand:
sed -i s/"get().map(Root::from_rooted)"/"get_rooted()"/g *.rs

Second part done manually after finding them with
git grep ".get().map("

Fixes 7929.