mbeddr.mpsutil: add a new language com.mbeddr.mpsutil.collections that provides an nset type

[alexanderpann]

A new language com.mbeddr.mpsutil.collections was added that adds support for a set type nset that use nodes as the values of the set. Equivalence of nodes is checked structurally. The hash code calculation is done for all properties and children and the first level of references. The runtime solution also contains a more general class EquivalenceHashSet to implement hash sets with arbitrary equals and hashcode methods. nset<> is also compatible with set<node> and can be used as its type instead.

Implementation note: I didn't use the built-in implementation like the matches statement and the StructuralNodeSet from MPS because it deals poorly with copied nodes. For example, a copy of a class sometimes is not structural equal to the class itself: https://youtrack.jetbrains.com/issue/MPS-37894/matches-operation-not-working-for-type-variables

[HeikoBecker]

Thanks for the extension. This is really useful.

Here are some preliminary technical findings. I am not yet done with a full review:

• Do we need this constructor? It looks dangerous since it may change the behavior of the underlying set.
  
• Can you add a test that NodeEquivalence.equals(a,b) -> NodeEquivalence.hash (a) == NodeEquivalence.hash (b) for consistency of hashCode with equals?
• This test seems duplicated:
  
• Nitpick, but I would prefer removing one element and testing for size equal 1:
  
• Why was equals replaced with :eq: in only this test: