Refactor to contracts engine

[waterlink]

Allows to be as non-intrusive as possible, moving all the functionality effectively to only one method .__contracts_engine (which has rich object(s) inside).

Still needs some cleanup:

• refactor MethodHandler#handle.
• remove Eigenclass.
• remove include Contracts::Module since now they get all the needed stuff properly.
• try to reduce amount of public methods on Engine. Or at least mark some of them as _pseudo_private. There is a probability, that some of them could be moved to protected (interaction between Engine and EigenclassEngine).
• add documentation in new code

closes #138

[waterlink]

@egonSchiele @sfcgeorge This one is not finished yet, but can you take a look and tell if you like the direction where this PR is going?

[waterlink]

just eigenclass_engine here

[betamatt]

❤️

[egonSchiele]

Works for me. Also happy that Contracts::Module can be removed! Since you are working on this, could you please add comments on all the methods? That will make it easy to modify this code next time...it is pretty complex.

[waterlink]

Finally green on CI. @egonSchiele can you take a look?

[egonSchiele]

Looks great. Thank you for adding all the comments!

[egonSchiele]

Weirdly, the change did make contracts a little slower.

Old:

                                     user     system      total        real
testing add                      0.430000   0.000000   0.430000 (  0.435320)
testing contracts add            3.540000   0.000000   3.540000 (  3.547737)

New:

                                     user     system      total        real
testing add                      0.430000   0.000000   0.430000 (  0.435508)
testing contracts add            3.950000   0.000000   3.950000 (  3.960544)

Not a meaningful slowdown.