Fix Lwt_react.S.limit and Lwt_react.E.limit.
Fix deprecated Lazy.lazy_from_fun usage.
Bump OCaml dependency to 4.01.

This removes all version-related preprocessing from Lwt.
Remove deprecated lwt.top library.
Remove deprecated lwt.text library.
Remove deprecated Lwt_signal and Lwt_event modules.
Remove deprecated Lwt_util module.
Remove deprecated lwt.extra library.
Replace all "#if windows" with "if Sys.win32".

Sys.win32 is available since 4.01 and is optimized into nothing
by ocamlopt. With ocamlc, there is a tiny startup time penalty.
Remove all usage of HAVE_* feature test macros in OCaml code.
Make primitives which accept () also accept a single argument in C.

Without this, OCaml and C assume different ABIs. While this may
not necessarily lead to crashes, it's still incorrect.
Get rid of all instances of "#if windows" that refer to primitives.

All "raise Not_available" calls are effectively folded into
the corresponding C stubs.
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.
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.
Use Lwt.Infix and Lwt.return_* wherever appropriate.
Update manual.