Support non-monotonic modifications in .olean files

Originally from module.cpp (RIP)

• Persistent set_option. We want to be able to store the option settings in .olean files.
  The main issue is conflict between imported modules. That is, each imported module wants to
  set a particular option with a different value. This can create counter-intuitive behavior.
  Consider the following scenario

  • A.olean : sets option foo to true
  • B.olean : imports A.olean
  • C.olean : sets option foo to false
  • We create D.lean containing the following import clause:

    import B C A
    

    The user may expect that foo is set to true since A is the last module to be imported,
    but this is not the case. B is imported first, then A (which sets option foo to true),
    then C (which sets option foo to false), the last import A is skipped since A has already
    been imported, and we get foo set to false.

  To address this issue we consider a persistent option import validator. The validator
  signs an error if there are two direct imports which try to set the same option to different
  values. For example, in the example above, B and C are conflicting, and an error would
  be signed when trying to import C. Then, users would have to resolve the conflict by
  creating an auxiliary import. For example, they could create the module C_aux.lean containing

  import C
  set_option persistent foo true
  

  and replace import B C A with import B C_aux A

• Removing attributes. The validation procedure for persistent options can be extended to attribute deletion. In the latest version, we can only locally remove attributes. The validator for attribute deletion would sign an error if there are two direct imports where one adds an attribute [foo] to a declaration bla and the other removes it.

• Parametric attributes. This is not a missing feature, but a bug. In the current version, we have
  parametric attributes and different modules may set the same declaration with different parameter values.
  We can fix this bug by using an attribute validator which will check parametric attributes, or we can allow parametric attributes to be set only once. That is, we sign an error if the user tries to reset them.

The issue points to the former module.cpp entry point and to .olean import validation; start by locating the current code responsible for importing and validating .olean files. Done means defining and implementing conflict handling for persistent options, attribute removal, and parametric attributes, including the described conflicting-import cases.