Add PPX derivers for lattices of tuple and record types

[sim642]

This implements a big portion of #31 using my experimental ppx_easy_deriving library. The goal of the library is to provide a simpler ppx_type_directed_value-like interface with no performance penalty. It is a revival of my two-year-old ppx-lattice branch in more polished state.

This PR changes record-valued domains to use [@@deriving lattice] and also changes the implementation of Lattice.Prod to do so. Over time, the amount of Lattice.Prod usage could be reduced by using records with meaningful field names instead.

TODO

• [ ] Publish ppx_easy_deriving on opam.
• [ ] Fix opam pin in CI.
• [ ] Derive pretty_diff.