Drop OCaml < 4.08

[dbuenzli]

I think it's time to move away from 4.02 which was released in 2015, seven years ago. 4.08 is a reasonable version, one of which brought many stdlib improvement.

Here are a few arguments:

1. It would also to use cmdliner without that monkey dance.
2. I was looking at the use of Astring in the repo which I would like to remove as a direct dependency (Fpath unfortunately also depends on it but I'd like to drop that dep there at some point too and/or remove it here too). Most of the uses of Astring are to work around the String.$(op) vs String.ascii_$op warnings (introduced 4.03). Other than that we have Astring.String.cut all of which use a single character and can be replaced by String.split_on_char (introduced 4.04). After that it only Astring.String.is_{prefix,affix} remains which we can easily define in the repo itself.
3. I'm pretty sure the 4.02 cmt[i] files were broken in some way.

[jonludlam]

As I mentioned in #873:

"I'd quite like to see the features we know we want for the ocaml website to land before we drop support for the older versions - so for example the 'uses' work that will be used to drive search. That way, once we've got those bits and pieces in we can just have a maintenance branch that supports 4.02-4.07 that will hopefully be easy to manage, and we won't have to backport that sort of relatively large feature.

Of course, if it actually does become more of a burden then we can revisit this, but so far keeping our lower bound at 4.02 has only been a case of a few minor shims here and there."