Should the new module name be Datatypes to match Datarepr? (or Datadesc? Labeldesc?)

Overall, I agree that detaching this module from Types make sense :

• the field and constructor descriptions are not types
• they are not used elsewhere in the Types modules
• they don't need to be privy to Types internal.

I would propose to move the two functions of Btypes label_is_poly and cstr_type_path to the new module to remove the Btype->Datatypes dependency.

Similarly, I would rather avoid opening the full module in the mli for Ctypes andTypecore (and possibly Datarepr) because the new module types are only used once or twice in the interface.

I applied your recommendations and rebased.

Changes:

• Btype.cstr_type_path is now Data_types.cstr_res_type_path
• I removed Btype.label_is_poly (I was not too exited at adding a dependency from Data_types (mostly a types-only module) to Btype (a types-and-logic module))
• I removed a two of the global open you mentioned (remark: reformating .mli lines to avoid 80 columns is not fun)

I prefer Data_types to Datatypes, because I can see a use for other Foo_types module, for example my in-progress PR has a Head_shape_types module, and Headshapetypes would be ugly. We could change Datadescr into something else, in time (never).

Thanks! I will fix the typos, massage the history (removing the opens could go in the first commit), merge, and rebase #13467.