Support -opaque

[rgrinberg]

Compiling cmx's with -opaque increases the effectiveness of incremental compilation at the expense of the quality of the generated code:

  -opaque  Does not generate cross-module optimization information
     (reduces necessary recompilation on module change)

dune should allow users to make this trade-off.

We're not entirely sure if passing the flag is enough, and if we also need to hide the module [1].

cc @talex5 who requested this feature

[1] #489 (comment)

[emillon]

Can this be closed now that #1079 has been merged or is hiding still relevant?

[rgrinberg]

Yup, this can be closed as well.