support opam 2.0 and dune

[avsm]

The Lwt documentation now recommends bheap to replace the Lwt_pqueue module. This PR correspondingly updates the metadata of the library to use similar tools to Lwt:

• update metadata to opam 2.0 format
• build and install with dune (which allows monorepo workflows as well as the standard opam one)

The opam metadata seems to point to another repository though, so I've preserved that but can fix it if it's meant to be backtracing/bheap.

I also fixed a deprecation warning on Array.make (#1) as part of this fix. I need to check what version that was introduced in to put a minimum constraint on the ocaml package.