Add `val_modalities`

[riaqn]

This PR allows values in structures to have modalities.

[riaqn]

I did most of the suggestions by @lpw25 except the following:

• It was suggested that Mode.Modality.Value.Const.to_list instead of returning the list of atom modalities, it should return a record of atom modalities, each field corresponding to an axis. I argue that the current interface allows printtyp.ml to not mention specific axes.
• It was suggested that val infer: mmode:Value.lr -> mode:Value.l -> t can split the argument mmode:lr into a left and a right corresponding to monadic and comonadic fragments. The reality is more messy. In short, the monadic fragments needs both left and right for better types. See cross in mode.ml.

[riaqn]

This is now ready for review. Please review the whole PR in one go - the commits are for my own tracking.

[riaqn]

The feature needs to be enabled by -extension mode_alpha. When it's disabled:

• User cannot write val f : t @@ m - will trigger error.
• When user write struct let f = ... end, the inferred modalities are fixed to be identity.
• When user refers to cmi built with -extension mode_alpha, the modalities in there will work as usual.

The above decision is driven by the example:

module M = struct let f x = x end
module M' = struct let (f @ nonportable) x = x end

For early adopters, the printed module types should be different; And that means the first should read sig val f : 'a -> 'a @@ portable while the second doesn't. However, for a normal user, finding val_modalities popping out from nowhere will be confusing. So we will use -extension mode_alpha to switch the behaviour.