Get rid of all instances of "#if windows || android".

pa_optcomp is henceforth not needed for building.
Add Lwt_config to InternalModules of lwt.unix.
Fix deprecated Lazy.lazy_from_fun usage.
Remove deprecated lwt.extra library.
Remove pa_optcomp, as it is not used anymore.
Remove pa_lwt syntax from libraries and tests.
Make camlp4 optional.
Make flag(ppx) default to flag(all) and update README.