add revery

[joprice]

This is an initial version of a Revery derivation. It currently works only for macos. It relies on multiple patched repos that are not defined along with the top-level ocaml derivations, as they are specific to Revery. I happened to be using ocaml 5.0 when building this, so there are several patches to get libs work with >= 5.x. If this should be made compatible with 4.x, I'll test that and make them optional or tweak them where necessary. There is also an issue with static vs dynamic compilation I need to investigate, where not all libs are built statically. I noticed some of the dune files in Revery have two rules for static and dynamic, so perhaps both varieties should be made available.