support ocaml >= 4.08

[vogler]

ppx_import blocks the upgrade to ocaml 4.08, track ocaml-ppx/ppx_import#41.
We use ppx_import to import Cil types to generate to_yojson and show with ppx_deriving, see https://github.com/goblint/analyzer/blob/master/src/util/deriving.ml#L22.
Alternatively we could maybe include our fork https://github.com/goblint/cil as a git submodule to have direct access to the types. However, I guess, it should then not be installed as an opam package but be built together with goblint.

[fpottier]

Just a comment to say it would be good to have ppx_import work for OCaml 4.08 and beyond. It can be really useful in conjunction with ppx_deriving, visitors, etc.

[michael-schwarz]

The ppx_import folks seem to be working on adding support for 4.08 now 🎉 ocaml-ppx/ppx_import#46

[michael-schwarz]

Their fix solves the problem with ppx_import.

Unfortunately, this leaves us with ppxx (which ppx_monadic relies on) still blocking support for 4.08.

[vogler]

I only used ppx_monadic in src/extract/ to have a more concise syntax.
There are several such libraries available if you search for monad in the packages.
OCaml 4.08 also introduced definable syntax for monads and applicatives (see here), however, it seems like there is no ;-sequencing.
Next best thing seemed to be ppx_let but it somehow constrains ppx_import via some dependency if I try to install it: downgrade ppx_import 1.6.2 to 1.5.
I then tried ocaml-monadic which seems to work and has not as nice but reasonable syntax. I'll adjust the files later.