Windows support

[panglesd]

With the release of opam 2.2, it seems important that odoc supports windows.

#1153 added windows to the CI (thanks!). Just the CI shows a few things:

• Tests are broken on windows:
  • The generation of dune files is broken as it uses windows' \ instead of dune's / in paths
  • The output in cram test differ due to windows' \ versus Unix's / e.g. in odoc html-targets.
• The -P, -L, --parent-id options seems to have problem with path separator too.

There might be (many) other issues not listed here.