Revive Coq 8.4

[Reilithion]

Commit 8df0ca2 removed the Coq 8.4 package. My colleagues and I still depend on that version of Coq for research. We do intend to eventually port our work to a newer version of Coq, but for now, having to use an old nixpkgs or have our own overlay to build our project would be inconvenient.

If having Coq 8.4 in nixpkgs isn't too big a burden, it'd be nice to have it back.

[danbst]

cc @Mic92

[stale]

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