This commit also removes Array.hmap.
Motivation: I wanted to use Array.hmap as an example in the paper, but
I found it would be too distracting to explain why we had `Array.hmap`
and `Array.map`.
cc @kha
@kha I renamed the homogeneous `map` to `hmap`, and added the
heterogeneous one as `map`. As soon as we add user-defined rewriting
rules, we will be able to replace `map` with `hmap` whenever the types
are the same.
