Drop support for 4.01

[aantron]

The main reason is to be able to use [@@ocaml.deprecated] to communicate API changes before/after some future major release. There are several other breaking changes that would benefit from this.

It's also a prerequisite for eventually removing the Camlp4 extension.

Please voice any objections.

[rgrinberg]

0 objections from me.

But Is there a way to check out which opam packages still use the obsolete camlp4 extension? Would be better to still warn people about this. Since the camlp4 removal itself will be without a deprecation cycle

[aantron]

I have a script on my machine that downloads all transitive dependents of Lwt found in OPAM, which I then look through (usually grep) for things like this. So, yes, I will warn at least the maintainers of those packages.

I will also mark lwt.syntax and the Camlp4 module as deprecated for a while, though I don't think I can get an automatic warning for this like for "normal" Lwt code. Haven't tried it yet.