Split code in different files

[sir4ur0n]

While reworking documentation as part of https://github.com/c-cube/qcheck/pull/109 I noticed that having the whole interface in a single file has several drawbacks:

• the interface declaration order is seriously constrained: the top level interface deserves user-oriented documentation, e.g. in Test I think that t and make are the most important bits (and thus should be the first presented to the user) but because of MLI typechecking, cell must be put first. There are plenty of other similar problems, e.g. having to present Tree and Shrink before Gen while Gen is arguably the most important module
• maintenance / navigation is a bit cumbersome on a ~1900 LOC file

At the very least I would imagine having a file for each module Test / TestResult / Gen / Tree / Shrink / Print / Fn / etc. (and there probably is a need to create a QCheck_internal to store all top level types and functions like arbitrary to be able to also change the declaration order for these)

[c-cube]

You can also declare, at the top of the mli, all types (opaque if needed); then list the submodules in the order that makes the most sense, binding type t = the_toplevel_forward_declaration. This way there is no need for a internal module.