Extract predicates for compiler versions

[emillon]

Instead of comparing on the version numbers, add some predicates in Context that describe the compiler behavior.

(as discussed in #1079)

[rgrinberg]

Rather than compiler I would suggest to use ocamlc.

Also, we might consider grouping these checks in a submodules and shortening the names, but this is a matter of taste.

[emillon]

Do you mean ocamlc in contrast to ocamlopt?

I think there might be some room to extract a module for the compiler version itself (int * int * int), I'll try to do that.

[rgrinberg]

Well we also have a C compiler that we sometimes use. But actually, we already have a val compiler function, so we might as well keep the compiler prefix

[emillon]

I extracted a module with an abstract type that does this. It will play nice with Comparable as well. Let me know what you think.

[ghost]

Looks good but I find the name Ocaml_compiler_version a bit long, what do you think of just Ocaml_version?

[emillon]

Sure, good idea! Just changed this and rebased.