Windows: Fix 'output_value: not a binary channel' when writing the AST to stdout

[bryphe]

Issue: I was building the latest ppx_deriving_yojson on Windows, and hit this error:

  # esy-build-package: building: @opam/ppx_deriving_yojson@opam:3.3
    # esy-build-package: pwd: C:\Users\bryph\.esy\3_\b\opam__s__ppx__deriving__yojson-opam__c__3.3-f1cb9869
    # esy-build-package: running: "dune" "build" "-p" "ppx_deriving_yojson" "-j" "4"
         ppxfind src/ppx_deriving_yojson.pp.ml (exit 2)
    (cd _build/default && C:\Users\bryph\.esy\3_\i\opam__s__ppxfind-opam__c__1.2-3fa788d0\bin\ppxfind.exe -legacy ppx_tools.metaquot --as-pp src/ppx_deriving_yojson.ml) > _build/default/src/ppx_deriving_yojson.pp.ml
    Fatal error: exception Failure("output_value: not a binary channel")
    error: command failed: "dune" "build" "-p" "ppx_deriving_yojson" "-j" "4" (exited with 1)
    esy-build-package: exiting with errors above...

  building @opam/ppx_deriving_yojson@opam:3.3@d196be77

Defect: ocaml-migrate-parsetree uses output_value without ensuring the channel is on binary m ode. On OSX/Linux, this is fine, because binary mode is the default. However, on Windows, we need to ensure the channel is in binary mode for either the stdout or file case.

The reason I hit this now is because ppx_deriving_yojson depends on ppxfind in its latest release, which depends on ocaml-migrate-parsetree.

Fix: Call set_binary_mode_out oc true prior to using the marshalling calls. This is a no-op on OSX/Linux, but will ensure the channel is in the correct mode on Windows.

cc @dra27

[ghost]

Looks good, thanks!

[ghost]

@bryphe, if you can confirm that this is enough to fix Windows compatibility, I'll prepare a new release.