Add Windows binaries

[Leonidas-from-XIV]

This PR uses the cross-compiling support implemented by @Alizter to cross-build Windows binaries on Nix. The resulting binary is then tested on "real" Windows.

Currently in draft as building a package fails as installing flexdll fails:

Error: entry
_build/_private/default/.pkg/flexdll.0.44-3397262cc27cdbe81fa917c723b35e88/source/default_amd64.manifest
in
_build/_private/default/.pkg/flexdll.0.44-3397262cc27cdbe81fa917c723b35e88/source/flexdll.install
does not exist
-> required by
   _build/_private/default/.pkg/flexdll.0.44-3397262cc27cdbe81fa917c723b35e88/target
-> required by
   _build/_private/default/.pkg/ocaml-base-compiler.5.5.0-7a6f8992bfeb9e42ff8bfecdc53fa9a6/target/cookie
-> required by loading the OCaml compiler for context "default"